cmd/go/internal/modload: avoid a network fetch when querying a valid semantic version

cmd/go/internal/modload: avoid a network fetch when querying a valid semantic version

Test this behavior incidentally in a test for ambiguous import errors.
(I rediscovered the error when writing the new test.)

For #32567
Updates #28806

Change-Id: I323f05145734e5cf99818b9f04d65075f7c0f787
Reviewed-on: https://go-review.googlesource.com/c/go/+/255046
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>