Track only files of known extensions when looking for module files
Track only files of known extensions when looking for module files

Fix #254

sdiehl/ghc,sdiehl/ghc,bgamari/shaking-up-ghc,sdiehl/ghc,izgzhen/hadrian,sdiehl/ghc,sdiehl/ghc,snowleopard/hadrian,sdiehl/ghc,snowleopard/shaking-up-ghc,sdiehl/ghc