cmd/go: do not treat branches with semantic-version names as releases

[bcmills]

#33558 (comment) reports a situation in which a v2.0.0 branch was created and subsequently removed, but proxy.golang.org cached a v2.0.0+incompatible release.

If someone creates a branch named v2.0.0, they probably intend for that branch to contain the development leading up to v2.0.0, not for that branch as first published to be the definitive final release of v2.0.0. The go command should not mistake the former for the latter.

CC @jayconrod @mattn @thepudds

[bcmills]

See #29731, which also involves confusion between branches and versions.

[hajimehoshi]

Any updates on this? I think this is a very scary issue, since if a collaborator created a vN branch deliberately or not once, the repository's versioning would be corrupted forever...