`cabal new-haddock` regression (setup: internal error when calculating transitive package dependencies.)

I originally noticed this issue when trying to [upgrade `singletons`' Travis setup](https://travis-ci.org/goldfirere/singletons/jobs/402254201#L4238) to use `cabal-install-head`. That is:

```
$ /opt/cabal/head/bin/cabal --version
cabal-install version 2.3.0.0
compiled using version 2.3.0.0 of the Cabal library
```

I've prepared a more minimal reproduction at https://github.com/RyanGlScott/cabal-gh5423. To reproduce the issue, do the following:

```
$ git clone https://github.com/RyanGlScott/cabal-gh5423
$ cd cabal-gh5423/
$ /opt/cabal/head/bin/cabal new-haddock cabal-gh5423:tests
Resolving dependencies...
Build profile: -w ghc-8.4.3 -O1
In order, the following will be built (use -v for more details):
- cabal-gh5423-0.1 (first run)
[1 of 1] Compiling Main ( /home/rgscott/Documents/Hacking/Haskell/cabal-gh5423/dist-newstyle/build/x86_64-linux/ghc-8.4.3/cabal-gh5423-0.1/setup/setup.hs, /home/rgscott/Documents/Hacking/Haskell/cabal-gh5423/dist-newstyle/build/x86_64-linux/ghc-8.4.3/cabal-gh5423-0.1/setup/Main.o )
Linking /home/rgscott/Documents/Hacking/Haskell/cabal-gh5423/dist-newstyle/build/x86_64-linux/ghc-8.4.3/cabal-gh5423-0.1/setup/setup ...
Configuring cabal-gh5423-0.1...
Found dependency closure
Preprocessing library for cabal-gh5423-0.1..
Running Haddock on library for cabal-gh5423-0.1..
Haddock coverage:
0% ( 0 / 1) in 'CabalSandbox'
Missing documentation for:
Module header
Documentation created:
/home/rgscott/Documents/Hacking/Haskell/cabal-gh5423/dist-newstyle/build/x86_64-linux/ghc-8.4.3/cabal-gh5423-0.1/doc/html/cabal-gh5423/index.html
Preprocessing test suite 'cabal-gh5423-tests' for cabal-gh5423-0.1..
Running Haddock on test suite 'cabal-gh5423-tests' for cabal-gh5423-0.1..
setup: internal error when calculating transitive package dependencies.
Debug info: []

cabal: Failed to build documentation for cabal-gh5423-0.1.
```

`cabal-gh5423` is a package with a library, test suite, and `custom-setup` script that looks like:

```haskell
module Main (main) where

import Distribution.PackageDescription
import Distribution.Simple
import Distribution.Simple.LocalBuildInfo
import Distribution.Simple.PackageIndex

main :: IO ()
main = defaultMainWithHooks simpleUserHooks
{ haddockHook = \pkg lbi hooks flags -> do
blah pkg lbi
haddockHook simpleUserHooks pkg lbi hooks flags
}

blah :: PackageDescription -> LocalBuildInfo -> IO ()
blah pkg lbi =
withLibLBI pkg lbi $ \_ libCLBI -> do
let libDeps = map fst $ componentPackageDeps libCLBI
case dependencyClosure (installedPkgs lbi) libDeps of
Left p -> p `seq` putStrLn "Found dependency closure"
Right _ -> error "Broken dependency closure"
```

(In reality, `singletons` actually uses the computed dependency closure for something useful, but I've elided this for the sake of keeping things simple.)

Clone the linked cabal-gh5423 reproduction and run the shown cabal new-haddock command first. Inspect its custom-setup setup.hs, especially haddockHook and the dependencyClosure call; done means documentation generation completes for the test suite without the internal transitive-dependency error.

Written by the indexing model from the issue text.