path: avoid import of strings

Pushing path lower in the hierarchy, to allow path < io/fs < os
in the io/fs prototype. But this change is worth doing even if io/fs
is not accepted.

Change-Id: Id51b3a638167ca005dadfb9b730287e518ec12a8
Reviewed-on: https://go-review.googlesource.com/c/go/+/243904


Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Rob Pike <r@golang.org>