Avoid treating `uv add -r` as `--raw-sources` (#6287)
## Summary
I suspect this was added because there's no way for users to pass (e.g.)
`--tag`, so the references are ambiguous. I think it's better to write
them as `rev` than to fail, though. It's just less efficient when we
fetch.
Closes https://github.com/astral-sh/uv/issues/6276.
Closes https://github.com/astral-sh/uv/issues/6275.
