If the replacements specify one or more versions, we choose the latest
(for consistency with the QueryPackage path, with resolves the latest
version from upstream).

Otherwise, we synthesize a pseudo-version with a zero timestamp and an
appropriate major version.

Fixes #26241

RELNOTE=yes

Change-Id: I14b4c63858c8714cc3e1b05ac52c33de5a16dea9
Reviewed-on: https://go-review.googlesource.com/c/152739Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>