Invalidate `uv.lock` if registry sources are removed (#6026)
## Summary
Now, if you resolve against a registry, then swap it out for another, we
won't reuse the lockfile. (If you don't provide any registry
configuration, then we won't enforce this, so that `uv lock --index-url
foo` and `uv lock` is stable.)
Closes https://github.com/astral-sh/uv/issues/5920.
