cmd/go: relax validation for replacements for gopkg.in paths
The 'go' command normally requires the 'go.mod' files for replacement
modules to have a major version compatible with the module they are
replacing.
However, prior to CL 206761, the 'go' command erroneously allowed
unversioned paths (which imply major version 0 or 1) to replace
'gopkg.in' paths with any major-version suffix.
An analysis of proxy.golang.org suggests that these replacements,
while uncommon, are not unheard-of. Rather than breaking the modules
that rely on them, we will continue to allow the erroneous replacement
paths for this particular pairing.
Updates #34254
Change-Id: Icb4e745981803edaa96060f17a8720a058219ab1
Reviewed-on: https://go-review.googlesource.com/c/go/+/212105
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Jay Conrod <jayconrod@google.com>