Don't expose hir::Path out of hir

Don't expose hir::Path out of hir

Conjecture: it's impossible to use hir::Path *correctly* from an IDE.

I am not entirely sure about this, and we might need to add it back at
some point, but I have to arguments that convince me that we probably
won't:

* `hir::Path` has to know about hygiene, which an IDE can't set up
  properly.

* `hir::Path` lacks identity, but you actually have to know identity
  to resolve it correctly