cmd/go: ignore retracted versions when converting revisions to versions

When a module author retracts a version, the go command should act as
if it doesn't exist unless it's specifically requested.

When converting a revision to a version, we should ignore tags for
retracted versions. For example, if the tag v1.0.0 is retracted, and
branch B points to the same revision, we should convert B to a
pseudo-version, not v1.0.0. Similarly, if B points to a commit after
v1.0.0, we should not use v1.0.0 as the base; we can use an earlier
non-retracted tag or no base.

Fixes #41700

Change-Id: Ia596b05b0780e5acfe6616a04e94d24bd342fbae
Reviewed-on: https://go-review.googlesource.com/c/go/+/261079


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