Replace PyPI-internal Hashes representation with flat vector (#2925)

## Summary

Right now, we have a `Hashes` representation that looks like:

```rust
/// A dictionary mapping a hash name to a hex encoded digest of the file.
///
/// PEP 691 says multiple hashes can be included and the interpretation is left to the client.
#[derive(Debug, Clone, Eq, PartialEq, Default, Deserialize)]
pub struct Hashes {
    pub md5: Option<Box<str>>,
    pub sha256: Option<Box<str>>,
    pub sha384: Option<Box<str>>,
    pub sha512: Option<Box<str>>,
}
```

It stems from the PyPI API, which returns a dictionary of hashes.

We tend to pass these around as a vector of `Vec<Hashes>`. But it's a
bit strange because each entry in that vector could contain multiple
hashes. And it makes it difficult to ask questions like "Is
`sha256:ab21378ca980a8` in the set of hashes"?

This PR instead treats `Hashes` as the PyPI-internal type, and uses a
new `Vec<HashDigest>` everywhere in our own APIs.

crates/uv-cache/src/lib.rs

@@ -598,7 +598,7 @@ impl CacheBucket {
             Self::FlatIndex => "flat-index-v0",
             Self::Git => "git-v0",
             Self::Interpreter => "interpreter-v0",
-            Self::Simple => "simple-v6",
+            Self::Simple => "simple-v7",
             Self::Wheels => "wheels-v0",
             Self::Archive => "archive-v0",
         }