The are various motivation points:
- I'm not aware anyone third party running Cabal tests (maybe nixpkgs?)
- No need to do gen-extra-source-files, as Cabal-tests is not distributed
- No multiple rebuilds of Cabal-QuickCheck, Cabal-tree-diff
- Slightly smaller Cabal sdist (which is quite big still)

Fix #6615

[ci skip]

Makes things a little simpler in GHC's build system if libraries are in
the same directory as their name.

At the top level we now have cabal/ and cabal-install/

We currently have 39 different authors, all those with a significant number
of patches are listed as copyright holders in the LICENSE file.