Previously we would always “upgrade” to the semantically-highest
version, even if a newer compatible version exists.

That made certain classes of mistakes irreversible: in general we
expect users to address bad releases by releasing a new (higher)
version, but if the bad release was an unintended +incompatible
version, then no release that includes a go.mod file can ever have a
higher version, and the bad release will be treated as “latest”
forever.

Instead, when considering a +incompatible version we now consult the
latest compatible (v0 or v1) release first. If the compatible release
contains a go.mod file, we ignore the +incompatible releases unless
they are expicitly requested (by version, commit ID, or branch name).

Fixes #34165
Updates #34189

Change-Id: I7301eb963bbb91b21d3b96a577644221ed988ab7
Reviewed-on: https://go-review.googlesource.com/c/go/+/204440
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>