Heal cache entries with missing source distributions (#7559)

## Summary

`uv cache prune --ci` will remove the source distribution directory. If
we then need to build a _different_ wheel (e.g., you're building a
package that has Python minor version-specific wheels), we fail, because
we expect the source to be there.

Now, if the source is missing, we re-download it. It would be slightly
easier to just _ignore_ that revision, but that would mean we'd also
lose the already-built wheels -- so if you ran against many Python
versions, we'd continuously lose the cached data.

Closes https://github.com/astral-sh/uv/issues/7543.

## Test Plan

We can add tests, but they _need_ to build non-pure Python wheels, which
tends to be expensive...

For reference:

```console
$ cargo run venv --python 3.12
$ cargo run pip install mercurial==6.8.1 --verbose
$ cargo run cache prune --ci
$ cargo run venv --python 3.11
$ cargo run pip install mercurial==6.8.1 --verbose
```

I also did this with a local `.tar.gz` that I downloaded from PyPI.

crates/uv-cache/src/lib.rs

@@ -45,6 +45,11 @@ impl CacheEntry {
         Self(path.into())
     }
 
+    /// Return the cache entry's parent directory.
+    pub fn shard(&self) -> CacheShard {
+        CacheShard(self.dir().to_path_buf())
+    }
+
     /// Convert the [`CacheEntry`] into a [`PathBuf`].
     #[inline]
     pub fn into_path_buf(self) -> PathBuf {