Implement a --verify-hashes hash-checking mode (#4007)

## Summary

This is an alternative to `--require-hashes` which will validate a hash
if it's present, but ignore requirements that omit hashes or are absent
from the lockfile entirely.

So, e.g., transitive dependencies that are missing will _not_ error; nor
will dependencies that are included but lack a hash.

Closes https://github.com/astral-sh/uv/issues/3305.

crates/uv-requirements/src/source_tree.rs

@@ -182,7 +182,8 @@ impl<'a, Context: BuildContext> SourceTreeResolver<'a, Context> {
         let hashes = match self.hasher {
             HashStrategy::None => HashPolicy::None,
             HashStrategy::Generate => HashPolicy::Generate,
-            HashStrategy::Validate { .. } => {
+            HashStrategy::Verify(_) => HashPolicy::Generate,
+            HashStrategy::Require(_) => {
                 return Err(anyhow::anyhow!(
                     "Hash-checking is not supported for local directories: {}",
                     path.user_display()