Allow specifying packages for ambiguous modules

[tfausak]

Fixes #9.

Still working on tidying this up. I was surprised to find that the ghc package does not have a parser for PackageNames in it. I think the best place to get that from is Cabal-syntax.

[tfausak]

Note that this requires the PackageImports language extension, which is a little surprising. Anyway, here's a complete example. Starting with this module:

{-# OPTIONS_GHC -fplugin=Imp #-}
main = print Data.SemVer.initial

The import is ambiguous:

Main.hs:2:14: error: [GHC-45102]
    Ambiguous module name ‘Data.SemVer’.
    it was found in multiple packages:
    semver-0.4.0.1 semver-range-0.2.8
  |
2 | main = print Data.SemVer.initial
  |              ^^^^^^^^^^^^^^^^^^^

So we can enable this new feature to disambiguate:

{-# OPTIONS_GHC -fplugin=Imp
  -fplugin-opt=Imp:--package=Data.SemVer:semver #-}
main = print Data.SemVer.initial

But that bumps into the extension being required:

Main.hs:3:14: error: [GHC-10032]
    Package-qualified imports are not enabled
    Suggested fix: Perhaps you intended to use PackageImports
  |
3 | main = print Data.SemVer.initial
  |              ^^^^^^^^^^^^^^^^^^^

So we can add the extension:

{-# LANGUAGE PackageImports #-}
{-# OPTIONS_GHC -fplugin=Imp
  -fplugin-opt=Imp:--package=Data.SemVer:semver #-}
main = print Data.SemVer.initial

And now it works:

Version {_versionMajor = 0, _versionMinor = 0, _versionPatch = 0, _versionRelease = [], _versionMeta = []}

[tfausak]

🤔 I wonder if Imp can automatically enable the PackageImports extension when required.