Reduce duplication in file resolver functions

[markus1189]

DRY up file resolver functions that only differ whether it's a file or directory by introducing internal helper functions.

[snoyberg]

Thanks! I'll leave to @chrisdone to review and comment on.

[markus1189]

Hope I fixed the build error due to missing <$> import, I just replaced it with fmap instead of adding CPP.

[chrisdone]

Looks good!

[markus1189]

There where still some missing Functor constraints for ghc < 7.10 ... I added them, let's see what travis says this time

[chrisdone]

I already fixed it on master, no worries.

[markus1189]

Awesome, funny how just using liftM in this case is better than fmap =)