cmd/go/internal/modget: split resolveCandidates into two methods

It turns out that the existing call sites of the resolveCandidates
method pass only *either* a slice of queries or a slice of upgrades
(never both), and the behaviors triggered by the two parameters don't
overlap much at all. To clarify the two different operations, split
them into two separate methods.

For #36460

Change-Id: I64651637734fd44fea68740a3cdfbacfb73c19b6
Reviewed-on: https://go-review.googlesource.com/c/go/+/289697


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