Expose getPath

While working on https://github.com/haskell/filepath/pull/250 I realized I need to inline insane amounts of code. Then I found out that directory basically already has what we need in internal modules. My understanding is that this would fit well in directory API, since it also deals with XDG. So PATH doesn't seem to be far fetched. This would also allow me to deprecate the [problematic](https://github.com/haskell/filepath/issues/251) `getSearchPath` in filepath.