cmd/go/internal/modload: return a module-only result from QueryPattern

This allows a single QueryPattern call to resolve a path that could be
either a package or a module. It is important to be able to make a
single QueryPattern call — rather than a QueryPattern followed by a
Query for the specific module path — to provide appropriate fallback
behavior: if the proxy returns package results but does not contain a
module result, we don't want to fall back to the next proxy to look
for the (probably-nonexistent) module.

For #37438

Change-Id: I419b8bb3ab4565f443bb5cee9a8b206f453b9801
Reviewed-on: https://go-review.googlesource.com/c/go/+/266657


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