Refine the fix for requiring Cabal > 1.20 for Setup.hs

[dcoutts]

Going along with the existing approach of using a constraint rather than altering the deps of custom Setup.hs scripts, but make the constraint only apply to Cabal instances that are dependencies of Setup.hs scripts not all instances of Cabal.

Also rearrange things a little with a dedicated solver policy function for adding a min dep on Cabal versions for setup scripts.

This is a follow-up to #4051. @ezyang @dagit

@kosmikus @grayjay could you have a look at my use of setupPP and let us know if this will make sure the constraint applies to all setup deps. I'm not quite sure about the linking and whether this will catch all cases of Cabal instances occurring as deps of Setup scripts.

[mention-bot]

@dcoutts, thanks for your PR! By analyzing the history of the files in this pull request, we identified @dagit, @hvr and @ezyang to be potential reviewers.

[grayjay]

The use of setupPP looks right to me.

[dcoutts]

@grayjay thanks!

[ezyang]

OK, fixing this ended up being a lot easier than I thought. Looks nice. But that CI needs to pass. Fortunately it just looks like something silly:

 new_build_requires_Cabal_1_20.sh (ignoring stdout+stderr):                        FAIL (1.43s)
      Unexpected exit status.

      Exit status: ExitFailure 1
      Working directory: /tmp/cabal-install-test-17715

      <stdout> was:
      Warning:
      /tmp/cabal-install-test-17715/custom-setup-old-cabal/custom-setup-old-cabal.cabal:
      A package using section syntax must specify at least
      'cabal-version: >= 1.2'.
      Resolving dependencies...
      cabal: Could not resolve dependencies:
      trying: custom-setup-1.0 (user goal)
      next goal: custom-setup-setup.Cabal (dependency of custom-setup-1.0)
      rejecting: custom-setup-setup.Cabal-1.18.1.5/installed-647... (constraint from
      minimum version of Cabal used by Setup.hs requires >=1.20)
      rejecting: custom-setup-setup.Cabal-1.24.0.0,
      custom-setup-setup.Cabal-1.22.8.0, custom-setup-setup.Cabal-1.22.7.0,
      custom-setup-setup.Cabal-1.22.6.0, custom-setup-setup.Cabal-1.22.5.0,
      custom-setup-setup.Cabal-1.22.4.0, custom-setup-setup.Cabal-1.22.3.0,
      custom-setup-setup.Cabal-1.22.2.0, custom-setup-setup.Cabal-1.22.1.1,
      custom-setup-setup.Cabal-1.22.1.0, custom-setup-setup.Cabal-1.22.0.0,
      custom-setup-setup.Cabal-1.20.0.4, custom-setup-setup.Cabal-1.20.0.3,
      custom-setup-setup.Cabal-1.20.0.2, custom-setup-setup.Cabal-1.20.0.1,
      custom-setup-setup.Cabal-1.20.0.0 (conflict: custom-setup =>
      custom-setup-setup.Cabal<1.20)
      rejecting: custom-setup-setup.Cabal-1.18.1.7,
      custom-setup-setup.Cabal-1.18.1.6, custom-setup-setup.Cabal-1.18.1.5,
      custom-setup-setup.Cabal-1.18.1.4, custom-setup-setup.Cabal-1.18.1.3,
      custom-setup-setup.Cabal-1.18.1.2, custom-setup-setup.Cabal-1.18.1.1,
      custom-setup-setup.Cabal-1.18.1, custom-setup-setup.Cabal-1.18.0,
      custom-setup-setup.Cabal-1.16.0.3, custom-setup-setup.Cabal-1.16.0.2,
      custom-setup-setup.Cabal-1.16.0.1, custom-setup-setup.Cabal-1.16.0,
      custom-setup-setup.Cabal-1.14.0, custom-setup-setup.Cabal-1.12.0,
      custom-setup-setup.Cabal-1.10.2.0, custom-setup-setup.Cabal-1.10.1.0,
      custom-setup-setup.Cabal-1.10.0.0, custom-setup-setup.Cabal-1.8.0.6,
      custom-setup-setup.Cabal-1.8.0.4, custom-setup-setup.Cabal-1.8.0.2,
      custom-setup-setup.Cabal-1.6.0.3, custom-setup-setup.Cabal-1.6.0.2,
      custom-setup-setup.Cabal-1.6.0.1, custom-setup-setup.Cabal-1.4.0.2,
      custom-setup-setup.Cabal-1.4.0.1, custom-setup-setup.Cabal-1.4.0.0,
      custom-setup-setup.Cabal-1.2.4.0, custom-setup-setup.Cabal-1.2.3.0,
      custom-setup-setup.Cabal-1.2.2.0, custom-setup-setup.Cabal-1.2.1,
      custom-setup-setup.Cabal-1.1.6 (constraint from minimum version of Cabal used
      by Setup.hs requires >=1.20)
      fail (backjumping, conflict set: custom-setup, custom-setup-setup.Cabal)
      Dependency tree exhaustively searched.
      die: Expect constraint failure

      ```

[hvr]

I've rebased (since the PR was created, the newly rewritten testsuite was merged) & fixed up the unit-test; I'll merge this as soon as Travis is done (and all is green).