Discover modules from preprocessors

[tfausak]

This fixes #14. See also #16.

These are documented by Cabal here: https://cabal.readthedocs.io/en/3.10/cabal-package.html#modules-and-preprocessors

• c2hs: *.chs
• cpphs: *.cpphs
• GreenCard: *.gc
• Haskell: *.hs and *.lhs
• Backpack: *.hsig and *.lhsig
• hsc2hs: *.hsc
• Happy: *.ly and *.y
• Alex: *.x

[tfausak]

For posterity, here's where (I think) Cabal defines which extensions it looks for:

https://github.com/haskell/cabal/blob/2acae632a0d625cd5b54aeb257fc83188344fa03/Cabal/src/Distribution/Simple/SrcDist.hs#L543 https://github.com/haskell/cabal/blob/2acae632a0d625cd5b54aeb257fc83188344fa03/Cabal/src/Distribution/Simple/PreProcess.hs#L909

There are no extensions there that we've missed. There is *.hs-boot and *.lhs-boot, but those always have a corresponding source file so they don't need to be discovered.