cmd/go: refactor modload.CheckRetractions

Extract queryLatestVersionIgnoringRetractions, which returns the
version we should load retractions and deprecations from. This will be
shared with CheckDeprecations.

Rename ShortRetractionRationale to ShortMessage. This will be used to
shorten deprecation warnings as well.

For #40357

Change-Id: Ic1e0c670396bdb3bd87c7a97cf2b14ca58ea1d80
Reviewed-on: https://go-review.googlesource.com/c/go/+/306332


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