Create helper functions for the exported URI functions to test
the logic that isn't OS-specific (filepath.{To,From}Slash is the OS-specific part).
Also add helpers to determine is a file or URI path is Windows-specific.
Change-Id: I6ba5119424ad5edcd59b946276e4268b2525505f
Reviewed-on: https://go-review.googlesource.com/c/153867
Reviewed-by: Ian Cottrell <iancottrell@google.com>
