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.

crates/uv-dispatch/src/lib.rs

@@ -21,7 +21,7 @@ use uv_configuration::{BuildKind, ConfigSettings, NoBinary, NoBuild, Reinstall,
 use uv_installer::{Downloader, Installer, Plan, Planner, SitePackages};
 use uv_interpreter::{Interpreter, PythonEnvironment};
 use uv_resolver::{FlatIndex, InMemoryIndex, Manifest, Options, Resolver};
-use uv_types::{BuildContext, BuildIsolation, EmptyInstalledPackages, InFlight, RequiredHashes};
+use uv_types::{BuildContext, BuildIsolation, EmptyInstalledPackages, HashStrategy, InFlight};
 
 /// The main implementation of [`BuildContext`], used by the CLI, see [`BuildContext`]
 /// documentation.
@@ -134,7 +134,6 @@ impl<'a> BuildContext for BuildDispatch<'a> {
     async fn resolve<'data>(&'data self, requirements: &'data [Requirement]) -> Result<Resolution> {
         let markers = self.interpreter.markers();
         let tags = self.interpreter.tags()?;
-        let hashes = RequiredHashes::default();
         let resolver = Resolver::new(
             Manifest::simple(requirements.to_vec()),
             self.options,
@@ -144,7 +143,7 @@ impl<'a> BuildContext for BuildDispatch<'a> {
             self.client,
             self.flat_index,
             self.index,
-            &hashes,
+            &HashStrategy::None,
             self,
             &EmptyInstalledPackages,
         )?;
@@ -178,9 +177,6 @@ impl<'a> BuildContext for BuildDispatch<'a> {
             venv.root().display(),
         );
 
-        // Don't enforce hashes for build dependencies.
-        let hashes = RequiredHashes::default();
-
         // Determine the current environment markers.
         let tags = self.interpreter.tags()?;
 
@@ -197,7 +193,7 @@ impl<'a> BuildContext for BuildDispatch<'a> {
             site_packages,
             &Reinstall::None,
             &NoBinary::None,
-            &RequiredHashes::default(),
+            &HashStrategy::None,
             self.index_locations,
             self.cache(),
             venv,
@@ -226,7 +222,8 @@ impl<'a> BuildContext for BuildDispatch<'a> {
             vec![]
         } else {
             // TODO(konstin): Check that there is no endless recursion.
-            let downloader = Downloader::new(self.cache, tags, &hashes, self.client, self);
+            let downloader =
+                Downloader::new(self.cache, tags, &HashStrategy::None, self.client, self);
             debug!(
                 "Downloading and building requirement{} for build: {}",
                 if remote.len() == 1 { "" } else { "s" },