Allow linting Cabal files

[tfausak]

In theory it should be possible to use cabal check to lint *.cabal files. However there are some potential problems:

• cabal check only works when there is exactly one *.cabal file in the current directory. You can't pass in a file name. You also can't pass in file contents on STDIN.
• The output of cabal check is not machine readable.
• cabal check only works with package descriptions, but other files use Cabal syntax (like cabal.project).

It's possible that these issues will be addressed by https://github.com/haskell/cabal/issues/8211.

[tfausak]

This is effectively blocked by upstream. I can't use cabal check today, and I don't have the time to improve it or write my own tool.