cmd/go/internal/modget: warn about unmatched packages exactly once

Due to an inverted condition, we were emitting a "matched no packages"
warning twice in some cases and not at all in others.

For #41315

Change-Id: I472cd2d4f75811c8734852f2bdd7346f4c612816
Reviewed-on: https://go-review.googlesource.com/c/go/+/254819


Trust: Bryan C. Mills <bcmills@google.com>
Trust: Michael Matloob <matloob@golang.org>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>