Add support for URL requirements in `--generate-hashes` (#2952)
## Summary
This PR enables hash generation for URL requirements when the user
provides `--generate-hashes` to `pip compile`. While we include the
hashes from the registry already, today, we omit hashes for URLs.
To power hash generation, we introduce a `HashPolicy` abstraction:
```rust
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum HashPolicy<'a> {
/// No hash policy is specified.
None,
/// Hashes should be generated (specifically, a SHA-256 hash), but not validated.
Generate,
/// Hashes should be validated against a pre-defined list of hashes. If necessary, hashes should
/// be generated so as to ensure that the archive is valid.
Validate(&'a [HashDigest]),
}
```
All of the methods on the distribution database now accept this policy,
instead of accepting `&'a [HashDigest]`.
Closes #2378.
