path/filepath, internal/filepathlite: move parts of filepath to filepathlite

path/filepath, internal/filepathlite: move parts of filepath to filepathlite

The path/filepath package needs to depend on the os package to
implement Abs, Walk, and other functions. Move the implementation
of purely lexical functions from path/filepath into
internal/filepathlite, so they can be used by os and
other low-level packages.

Change-Id: Id211e547d6f1f58c82419695ff2d75cd6cd14a12
Reviewed-on: https://go-review.googlesource.com/c/go/+/566556
Reviewed-by: Behroz Karimpor <behrozkarimpor201@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>