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)

Rework verbosity parsing too and add tests.

Move Described functionality there, out of Cabal the library.
Similarly cabal-install doesn't contain Described instances,
they are only in tests.

Note all Described instances are implemented. This is just a start.