path/filepath: return cleaned path from Rel

path/filepath: return cleaned path from Rel

As the doc says, Rel should return a cleaned path.

Fixes #75763

Change-Id: Ic0f5a3b1da3cc4cf3c31fdb1a88ebcc4ea6f9169
Reviewed-on: https://go-review.googlesource.com/c/go/+/709675
Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Sean Liao <sean@liao.dev>
Auto-Submit: Sean Liao <sean@liao.dev>
Reviewed-by: Carlos Amedee <carlos@golang.org>