cabal-install-x.y.* should require Cabal-x.y.* for Custom-type builds

Something changed between 1.20.0.3 and 1.22.0.0, causing the Idris custom [`Setup.hs`](https://github.com/idris-lang/Idris-dev/blob/master/Setup.hs) to include invalid values.

In the `dist/build/autogen/Paths_idris.hs` obtained with 1.22.0.0, both `libdir` and `datadir` are wrong:

```
libdir = "/app/lib/$abi/idriskey"
datadir = "/app/share/$abi/idris-0.9.15.1"
```

Expected values, obtained with 1.20.0.3:

```
libdir = "/app/lib/x86_64-linux-ghc-7.8.4/idris-0.9.15.1"
datadir = "/app/share/x86_64-linux-ghc-7.8.4/idris-0.9.15.1"
```

See also https://github.com/idris-lang/Idris-dev/issues/1836.

Start with the Idris Setup.hs linked in the report and compare the generated dist/build/autogen/Paths_idris.hs values produced by Cabal 1.20.0.3 and 1.22.0.0. Trace how Custom-type builds derive libdir and datadir, using issue 1836 for context. Done means the generated paths use the expected architecture-specific values rather than literal $abi placeholders.

Written by the indexing model from the issue text.