Prune unused source distributions from the cache (#7112)
## Summary
This has bothered me for a while and should be fairly impactful for
users. It requires a weird implementation, since the
distribution-building crate depends on the cache, and so the prune
operation can't live in the cache, since it needs to access internals of
the distribution-building crate.
Closes https://github.com/astral-sh/uv/issues/7096.
