Potential use case: setting "repository_version" on the distribution freezes the registry

[dralley]

Setting repository_version on a Distribution would currently be a useless configuration, because requests for content not in the repo would still fall back to the proxy and modify the repository behind your back - functionally identical if not strictly worse than just setting repository in the first place.

What we could do instead is disable the fallback entirely if repository_version was set, turning the registry "frozen"

This would be simple to do with a tweak to get_rvc().