cmd/go: convert semver tags with metadata to pseudoversions

Some repositories include tags like 'v1.0.0-rc.1+oryOS.9'.

If we were to allow such tags, they could become ambiguous: semantic
versioning defines versions that differ only in metadata to have equal
precedence, so if someone added a tag 'v1.0.0-rc.1+other' at a
different commit, then the version 'v1.0.0-rc.1' would become
ambiguous.

However, we should still allow those tags to be used to resolve
versions, and since we can even parse the underlying semantic version,
we can at least use that as the basis for a unique (and well-ordered)
pseudo-version.

Fixes #31713

Change-Id: I5035f76d74ead6e786c04a368595cb5e42d36f91
Reviewed-on: https://go-review.googlesource.com/c/go/+/176905


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