Yes, you do have to keep your master clean. The version tagging thing is only intended to be used when you start depending on a new language feature not available in older versions of Go. For example, if your new code needs something that's only in Go 1.1, you could create a 1.0 tag right before making that change. But people using new versions of Go will get whatever is in master, not whatever is at that tag.

Yes, you can always clone explicitly into a path, but then we're not talking about the module system any more, are we? We're just talking about you copying around files locally. It's misleading to state this like it's the recommended or usual way to do things.