cmd: relocate search.MatchPattern to cmd/internal/pkgpattern

cmd: relocate search.MatchPattern to cmd/internal/pkgpattern

Relocate cmd/go's search.MatchPattern helper routine to a new package
in cmd/internal from its current location, as to allow it to be used
in other tools that accept package pattern command line flags. No
change in functionality along the way.

Updates #51430.

Change-Id: I726e974ccd66a055bb5a94497b36b8d68d47cad1
Reviewed-on: https://go-review.googlesource.com/c/go/+/432757
Reviewed-by: Bryan Mills <bcmills@google.com>