cmd/go/internal/modload: use replacements to resolve missing imports

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/152739


Reviewed-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>