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.
