Support Cabal 2.0

[dmalikov]

At the moment, Setup.hs depends on cabal < 1.25. Thus, ghc-mod cannot be installed via cabal-install > 1.25:

[1 of 1] Compiling Main             ( /Users/m/git/ghc-mod/dist-newstyle/build/x86_64-osx/ghc-8.2.1/ghc-mod-5.8.0.0/setup/setup.hs, /Users/m/git/ghc-mo
d/dist-newstyle/build/x86_64-osx/ghc-8.2.1/ghc-mod-5.8.0.0/setup/Main.o )

/Users/m/git/ghc-mod/dist-newstyle/build/x86_64-osx/ghc-8.2.1/ghc-mod-5.8.0.0/setup/setup.hs:85:60: error:
    • Couldn't match expected type ‘UnitId’
                  with actual type ‘PathTemplate’
    • In the third argument of ‘substPathTemplate’, namely
        ‘(progPrefix lbi)’
      In the expression:
        substPathTemplate (packageId pd) lbi (progPrefix lbi)
      In an equation for ‘progprefix’:
          progprefix = substPathTemplate (packageId pd) lbi (progPrefix lbi)
   |
85 |         progprefix = substPathTemplate (packageId pd) lbi (progPrefix lbi)
   |                                                            ^^^^^^^^^^^^^^
...

[DanielG]

I think almost all of the code in the currently released Setup.hs is not used anymore so in principle we could release a minor version that just removes that. However I'm working on GHC-8.2 support already and it'll be fixed as part of that either way so I'd prefer not to double the testing effort required. Unless there are a significant number of users that are using Cabal-2 together with GHC<8.2. I do think cabal-install should be able to fall back to an older Cabal versions (if one is installed) in that case though.

[DanielG]

Closing in favor of #900