Handle change tracking for pre-processor files / don't warn about them

[mgsloan]

In #479 (comment) , warnings are emitted for modules that are defined via preprocessor. Since module name -> file path resolution is used for knowing when to rebuild, I think there is likely a rebuild issue here.

Not sure how to unambiguously resolve module names to files that do not have .hs or .lhs extensions, though. A hack could be to assume that it is the correct file if the name matches, ignoring the extension, and if it's listed in extra-source-files as suggested by the warning text. Currently it seems like extra-source-files is not taken into account here. So, either the warning text should be fixed or the behavior should make it so that its suggestion works.

Repro is at https://github.com/adituv/protobuf/tree/testpreprocess

[adituv]

Perhaps there could be a stack.yaml field to list extensions there are custom preprocessors for?

[mgsloan]

Not a bad idea!