search.CleanPatterns now preserves backslash separators in absolute
paths in Windows. These had resulted in inconsistent error messages.

search.MatchPackagesInFS is now more accepting of patterns with
backslashes. It was inconsistent before.

Several tests are fixed to work with Windows (mostly to match slashes
or backslashes).

Fixes #25300

Change-Id: Ibbf9ccd145353f7e3d345205c6fcc01d7066d1c2
Reviewed-on: https://go-review.googlesource.com/c/go/+/206144
Run-TryBot: Jay Conrod <jayconrod@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>