Use separate path types for directories and files (#4285)

## Summary

This is what I consider to be the "real" fix for #8072. We now treat
directory and path URLs as separate `ParsedUrl` types and
`RequirementSource` types. This removes a lot of `.is_dir()` forking
within the `ParsedUrl::Path` arms and makes some states impossible
(e.g., you can't have a `.whl` path that is editable). It _also_ fixes
the `direct_url.json` for direct URLs that refer to files. Previously,
we wrote out to these as if they were installed as directories, which is
just wrong.

crates/uv-types/src/hash.rs

@@ -150,7 +150,8 @@ fn uv_requirement_to_package_id(requirement: &Requirement) -> Result<PackageId,
         }
         RequirementSource::Url { url, .. }
         | RequirementSource::Git { url, .. }
-        | RequirementSource::Path { url, .. } => PackageId::from_url(url),
+        | RequirementSource::Path { url, .. }
+        | RequirementSource::Directory { url, .. } => PackageId::from_url(url),
     })
 }