Closes #8440

* Changes needed for GenValidate and release.py to
  accommmodate new solver dependency
* Bumps bootstrap plans to modern GHC versions
* Update `validate.sh` and `release.py` to accommodate new solver dep.
* Update `Makefile` targets

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)

Previously this would work until CABAL_TESTSUITE_BDIR was needed, at
which point it would fail because it looked for things under
.../ghc-8.6/... while cabal v2-build actually put them in
.../ghc-8.6.5/...

Hard-coding "ghc" is unfortunate, but I didn't know if there was an easy
way to avoid it.  I also didn't want to over-complicate it, until it was
necessary.

validate.sh isn't tested in CI so...
[ci skip]

In recent versions of cabal-plan the --builddir= flag has become a
non-global flag; i.e. it needs to be placed after the sub-command rather
than before.

`validate.sh` isn't tested in CI so...
[ci skip]

- Add bunch of installed constraints to cabal.project.local.travis
- Fix compat in D.Compat.Directory
- Compat (process-1.1/ghc-7.6) in cabal-testsuite
- validate.sh improvements
- Warningless cabal-testsuite build

Resolve #5025

Now there's --help, so it's easier to use :)