Assertion failed in `invariant` in `Distribution.Simple.PackageIndex` from `mkPackageIndex`

`cabal install` is throwing the following error:

```
cabal: Distribution/Simple/PackageIndex.hs:163:30-35: Assertion failed
```

Line [163](https://github.com/haskell/cabal/blob/master/Cabal/Distribution/Simple/PackageIndex.hs#L163) is `mkPackageIndex`, which uses `invariant` at line [138](https://github.com/haskell/cabal/blob/master/Cabal/Distribution/Simple/PackageIndex.hs#L138).

```
invariant :: HasInstalledPackageId a => PackageIndex a -> Bool
invariant (PackageIndex pids pnames) =
map installedPackageId (Map.elems pids)
== sort
[ assert pinstOk (installedPackageId pinst)
| (pname, pvers) <- Map.toList pnames
, let pversOk = not (Map.null pvers)
, (pver, pinsts) <- assert pversOk $ Map.toList pvers
, let pinsts' = sortBy (comparing installedPackageId) pinsts
pinstsOk = all (\g -> length g == 1)
(groupBy (equating installedPackageId) pinsts')
, pinst <- assert pinstsOk $ pinsts'
, let pinstOk = packageName pinst == pname
&& packageVersion pinst == pver
]
```

I've modified this function to print out the two lists compared (which throws `False` to throw the "Assertion failed" error), i.e. the `map` and `sort`. There is one difference: the `sort` list has an additional element `InstalledPackageId ""`. The lists in full are:

For `map`:

```
[InstalledPackageId "",
InstalledPackageId ".fake.cmdargs-0.10.13",
InstalledPackageId ".fake.comonad-4.2.6",
InstalledPackageId ".fake.contravariant-1.3.1.1",
InstalledPackageId ".fake.cpphs-1.19",
InstalledPackageId ".fake.either-4.4.1",
InstalledPackageId ".fake.free-4.12.1",
InstalledPackageId ".fake.ghc-mod-0",
InstalledPackageId ".fake.haskell-src-exts-1.16.0.1",
InstalledPackageId ".fake.hlint-1.9.21",
InstalledPackageId ".fake.hscolour-1.23",
InstalledPackageId ".fake.monad-journal-0.7.1",
InstalledPackageId ".fake.polyparse-1.11",
InstalledPackageId ".fake.profunctors-5.1.1",
InstalledPackageId ".fake.semigroupoids-5.0.0.2",
InstalledPackageId ".fake.uniplate-1.6.12",
InstalledPackageId ".fake.void-0.7",
InstalledPackageId "Cabal-1.22.2.0-3936f53703ebda5ea195e67d7e9c3f8e",
InstalledPackageId "MonadRandom-0.4-b1143f1547bbab10e9fe85ad98d3dc37",
InstalledPackageId "StateVar-1.1.0.0-ada36b4807703e018c0794c69aec8a2f",
InstalledPackageId "ansi-terminal-0.6.2.1-e60ea8d9650d1a27f85b0c94a2ae426d",
InstalledPackageId "array-0.5.1.0-29bb26a0797af39b979b99b93e9d62fd",
InstalledPackageId "async-2.0.2-a598095e42153d2ceb887e406cdadecb",
InstalledPackageId "base-4.8.0.0-fc683f3bc9a20f2c5e9cfd686d3063fe",
InstalledPackageId "base-orphans-0.3.2-491aedcd4f6c26dafe87c9bb125d1e21",
InstalledPackageId "bifunctors-5-31a751fce70123b91d884e35e26e330b",
InstalledPackageId "bin-package-db-0.0.0.0-35c08c6cca5b7716f9cb710b071ab345",
InstalledPackageId "binary-0.7.3.0-c1bcaffb597b31afc4f9ebdda6e77954",
InstalledPackageId "builtin_rts",
InstalledPackageId "bytestring-0.10.6.0-57aa263f455e11099626dc3ad5d5ea63",
InstalledPackageId "cereal-0.4.1.1-3b610532bf14c7afae71d724ec697629",
InstalledPackageId "containers-0.5.6.2-c198d07e10a7e628be3224ecd2f965ac",
InstalledPackageId "data-default-0.5.3-a87ad971de5440e984b68e075ccea45b",
InstalledPackageId "data-default-class-0.0.1-3a048d82a8e68fb195fca91554d855e5",
InstalledPackageId "data-default-instances-base-0.0.1-ffab472c2e52a8edb76efd0bb772a3b0",
InstalledPackageId "data-default-instances-containers-0.0.1-28482c9b80d3d3b8d2b348efe8b74ac8",
InstalledPackageId "data-default-instances-dlist-0.0.1-2966709cad0f3592e01789323925aab7",
InstalledPackageId "data-default-instances-old-locale-0.0.1-dfe5215bfcbed690cb87cd437c6a0e6e",
InstalledPackageId "deepseq-1.4.1.1-2d3d22d845d91115f659d1905026d5b7",
InstalledPackageId "directory-1.2.2.0-d77665567792e0a58c62ffc852352d54",
InstalledPackageId "distributive-0.4.4-39cb7e20cf26fadc411bf1b2a94ae3a4",
InstalledPackageId "djinn-ghc-0.0.2.3-68d9291de929b3cb3044bb92c1e89d5d",
InstalledPackageId "djinn-lib-0.0.1.2-cc61fe161ed37b003a59103be888e586",
InstalledPackageId "dlist-0.7.1.1-94b618c591942a74d7954bc8eaeba690",
InstalledPackageId "exceptions-0.8.0.2-9a7923b7a246b9847ecf43c09f72a588",
InstalledPackageId "extra-1.2-955cbc84710c04207de4e5728aebb8a0",
InstalledPackageId "filepath-1.4.0.0-129f3fdd2b5de4f823a2641d7cf29327",
InstalledPackageId "ghc-7.10.1-c68fad7c073b01c18711eb22d66d9d8d",
InstalledPackageId "ghc-paths-0.1.0.9-11063b507da7cebbd8fcb8ea2e4d02ed",
InstalledPackageId "ghc-prim-0.4.0.0-361e8e2c9cf389d2a3e7a0e22f6c7443",
InstalledPackageId "ghc-syb-utils-0.2.3-0010a0acca060890bfdf6c796c4f470b",
InstalledPackageId "hashable-1.2.3.2-abaff392af18df3776edc3efe222e60f",
InstalledPackageId "hoopl-3.10.0.2-f6299a899ea4e758049dd187b46ff2f9",
InstalledPackageId "hpc-0.6.0.2-675064d54e8c1ba56c1ee5fa9960c2b0",
InstalledPackageId "integer-gmp-1.0.0.0-c95adc98e5a4d89e0ee310717e5e2c43",
InstalledPackageId "mmorph-1.0.4-d073ebc08ccb1af99a0eb38af289a1bd",
InstalledPackageId "mtl-2.2.1-ec1683bea2e61c9bd21815560f7f39b9",
InstalledPackageId "nats-1-0ef0c82e02fc5f1fa77c665b35287eb2",
InstalledPackageId "old-locale-1.0.0.7-c84f8967506c16ecca76a13cbb8d5b2c",
InstalledPackageId "old-time-1.1.0.3-2b7fae0b4c43d3674189446125d6a236",
InstalledPackageId "prelude-extras-0.4-8588e5cf44d181e520cf1a8a8ee409ac",
InstalledPackageId "pretty-1.1.2.0-819a82366d7bf09dc023f93dbe40c6b8",
InstalledPackageId "process-1.2.3.0-42d431fd80c0fb4e3f297f266822b9fd",
InstalledPackageId "random-1.1-cdf5743b87769f8096d3d60a999bff8b",
InstalledPackageId "semigroups-0.16.2.2-13a60cc8bba61c9d5a507a9d6800b40d",
InstalledPackageId "split-0.2.2-d604c6f9641ecbe6c002883f34f5a351",
InstalledPackageId "stm-2.4.4-b8ccbbe01dd6066dc57de099fd55acd9",
InstalledPackageId "syb-0.5.1-71724a5616d02c3e5fcc320898738c68",
InstalledPackageId "tagged-0.8.0.1-488e1e99b24c4da51e98516cce2daec9",
InstalledPackageId "template-haskell-2.10.0.0-44fef4903d5ebb1b4f10c81ce80bf2dc",
InstalledPackageId "temporary-1.2.0.3-419d183a4c5c86eda9eb7c729292c2da",
InstalledPackageId "text-1.2.1.1-c1348ccab9357bc56af672150021cd92",
InstalledPackageId "time-1.5.0.1-6bd680afd02660168e1f7ddbe7abb0bb",
InstalledPackageId "transformers-0.4.2.0-813aa0ed46c81874429c99c4bd62bf20",
InstalledPackageId "transformers-base-0.4.4-ac74f7840937f3d84a02cde1e5efb267",
InstalledPackageId "transformers-compat-0.4.0.4-0ef0c82e02fc5f1fa77c665b35287eb2",
InstalledPackageId "unix-2.7.1.0-432b92979da93055c3f3d8959cae075c",
InstalledPackageId "unordered-containers-0.2.5.1-5b6e12b25552e1466b6a6d96c24bae6b"]
```

For `sort`:

```
[InstalledPackageId "",
InstalledPackageId "",
InstalledPackageId ".fake.cmdargs-0.10.13",
InstalledPackageId ".fake.comonad-4.2.6",
InstalledPackageId ".fake.contravariant-1.3.1.1",
InstalledPackageId ".fake.cpphs-1.19",
InstalledPackageId ".fake.either-4.4.1",
InstalledPackageId ".fake.free-4.12.1",
InstalledPackageId ".fake.ghc-mod-0",
InstalledPackageId ".fake.haskell-src-exts-1.16.0.1",
InstalledPackageId ".fake.hlint-1.9.21",
InstalledPackageId ".fake.hscolour-1.23",
InstalledPackageId ".fake.monad-journal-0.7.1",
InstalledPackageId ".fake.polyparse-1.11",
InstalledPackageId ".fake.profunctors-5.1.1",
InstalledPackageId ".fake.semigroupoids-5.0.0.2",
InstalledPackageId ".fake.uniplate-1.6.12",
InstalledPackageId ".fake.void-0.7",
InstalledPackageId "Cabal-1.22.2.0-3936f53703ebda5ea195e67d7e9c3f8e",
InstalledPackageId "MonadRandom-0.4-b1143f1547bbab10e9fe85ad98d3dc37",
InstalledPackageId "StateVar-1.1.0.0-ada36b4807703e018c0794c69aec8a2f",
InstalledPackageId "ansi-terminal-0.6.2.1-e60ea8d9650d1a27f85b0c94a2ae426d",
InstalledPackageId "array-0.5.1.0-29bb26a0797af39b979b99b93e9d62fd",
InstalledPackageId "async-2.0.2-a598095e42153d2ceb887e406cdadecb",
InstalledPackageId "base-4.8.0.0-fc683f3bc9a20f2c5e9cfd686d3063fe",
InstalledPackageId "base-orphans-0.3.2-491aedcd4f6c26dafe87c9bb125d1e21",
InstalledPackageId "bifunctors-5-31a751fce70123b91d884e35e26e330b",
InstalledPackageId "bin-package-db-0.0.0.0-35c08c6cca5b7716f9cb710b071ab345",
InstalledPackageId "binary-0.7.3.0-c1bcaffb597b31afc4f9ebdda6e77954",
InstalledPackageId "builtin_rts",
InstalledPackageId "bytestring-0.10.6.0-57aa263f455e11099626dc3ad5d5ea63",
InstalledPackageId "cereal-0.4.1.1-3b610532bf14c7afae71d724ec697629",
InstalledPackageId "containers-0.5.6.2-c198d07e10a7e628be3224ecd2f965ac",
InstalledPackageId "data-default-0.5.3-a87ad971de5440e984b68e075ccea45b",
InstalledPackageId "data-default-class-0.0.1-3a048d82a8e68fb195fca91554d855e5",
InstalledPackageId "data-default-instances-base-0.0.1-ffab472c2e52a8edb76efd0bb772a3b0",
InstalledPackageId "data-default-instances-containers-0.0.1-28482c9b80d3d3b8d2b348efe8b74ac8",
InstalledPackageId "data-default-instances-dlist-0.0.1-2966709cad0f3592e01789323925aab7",
InstalledPackageId "data-default-instances-old-locale-0.0.1-dfe5215bfcbed690cb87cd437c6a0e6e",
InstalledPackageId "deepseq-1.4.1.1-2d3d22d845d91115f659d1905026d5b7",
InstalledPackageId "directory-1.2.2.0-d77665567792e0a58c62ffc852352d54",
InstalledPackageId "distributive-0.4.4-39cb7e20cf26fadc411bf1b2a94ae3a4",
InstalledPackageId "djinn-ghc-0.0.2.3-68d9291de929b3cb3044bb92c1e89d5d",
InstalledPackageId "djinn-lib-0.0.1.2-cc61fe161ed37b003a59103be888e586",
InstalledPackageId "dlist-0.7.1.1-94b618c591942a74d7954bc8eaeba690",
InstalledPackageId "exceptions-0.8.0.2-9a7923b7a246b9847ecf43c09f72a588",
InstalledPackageId "extra-1.2-955cbc84710c04207de4e5728aebb8a0",
InstalledPackageId "filepath-1.4.0.0-129f3fdd2b5de4f823a2641d7cf29327",
InstalledPackageId "ghc-7.10.1-c68fad7c073b01c18711eb22d66d9d8d",
InstalledPackageId "ghc-paths-0.1.0.9-11063b507da7cebbd8fcb8ea2e4d02ed",
InstalledPackageId "ghc-prim-0.4.0.0-361e8e2c9cf389d2a3e7a0e22f6c7443",
InstalledPackageId "ghc-syb-utils-0.2.3-0010a0acca060890bfdf6c796c4f470b",
InstalledPackageId "hashable-1.2.3.2-abaff392af18df3776edc3efe222e60f",
InstalledPackageId "hoopl-3.10.0.2-f6299a899ea4e758049dd187b46ff2f9",
InstalledPackageId "hpc-0.6.0.2-675064d54e8c1ba56c1ee5fa9960c2b0",
InstalledPackageId "integer-gmp-1.0.0.0-c95adc98e5a4d89e0ee310717e5e2c43",
InstalledPackageId "mmorph-1.0.4-d073ebc08ccb1af99a0eb38af289a1bd",
InstalledPackageId "mtl-2.2.1-ec1683bea2e61c9bd21815560f7f39b9",
InstalledPackageId "nats-1-0ef0c82e02fc5f1fa77c665b35287eb2",
InstalledPackageId "old-locale-1.0.0.7-c84f8967506c16ecca76a13cbb8d5b2c",
InstalledPackageId "old-time-1.1.0.3-2b7fae0b4c43d3674189446125d6a236",
InstalledPackageId "prelude-extras-0.4-8588e5cf44d181e520cf1a8a8ee409ac",
InstalledPackageId "pretty-1.1.2.0-819a82366d7bf09dc023f93dbe40c6b8",
InstalledPackageId "process-1.2.3.0-42d431fd80c0fb4e3f297f266822b9fd",
InstalledPackageId "random-1.1-cdf5743b87769f8096d3d60a999bff8b",
InstalledPackageId "semigroups-0.16.2.2-13a60cc8bba61c9d5a507a9d6800b40d",
InstalledPackageId "split-0.2.2-d604c6f9641ecbe6c002883f34f5a351",
InstalledPackageId "stm-2.4.4-b8ccbbe01dd6066dc57de099fd55acd9",
InstalledPackageId "syb-0.5.1-71724a5616d02c3e5fcc320898738c68",
InstalledPackageId "tagged-0.8.0.1-488e1e99b24c4da51e98516cce2daec9",
InstalledPackageId "template-haskell-2.10.0.0-44fef4903d5ebb1b4f10c81ce80bf2dc",
InstalledPackageId "temporary-1.2.0.3-419d183a4c5c86eda9eb7c729292c2da",
InstalledPackageId "text-1.2.1.1-c1348ccab9357bc56af672150021cd92",
InstalledPackageId "time-1.5.0.1-6bd680afd02660168e1f7ddbe7abb0bb",
InstalledPackageId "transformers-0.4.2.0-813aa0ed46c81874429c99c4bd62bf20",
InstalledPackageId "transformers-base-0.4.4-ac74f7840937f3d84a02cde1e5efb267",
InstalledPackageId "transformers-compat-0.4.0.4-0ef0c82e02fc5f1fa77c665b35287eb2",
InstalledPackageId "unix-2.7.1.0-432b92979da93055c3f3d8959cae075c",
InstalledPackageId "unordered-containers-0.2.5.1-5b6e12b25552e1466b6a6d96c24bae6b"]
```

Where is the additional `InstalledPackageId ""` element in the sorted list coming from?

Read Cabal/Distribution/Simple/PackageIndex.hs around mkPackageIndex and invariant, then reproduce the failure with cabal install while comparing the pids and pnames-derived lists. Done means identifying which package-index entry produces the extra empty InstalledPackageId and documenting the source of the mismatch.

Written by the indexing model from the issue text.