Fetch pyproject.toml from GitHub API (#10765)

## Summary

When resolving Git metadata, we may be able to fetch the metadata from
GitHub directly in some cases. This is _way_ faster, since we don't need
to perform many Git operations and, in particular, don't need to clone
the repo.

This only works in the following cases:

- The Git repository is public. Otherwise, I believe you need an access
token, which we don't have.
- The `pyproject.toml` has static metadata.
- The `pyproject.toml` has no `tool.uv.sources`. Otherwise, we need to
lower them... And, if there are any paths or workspace sources, that
requires an install path (i.e., we need the content on-disk).
- The project is in the repo root. If it's in a subdirectory, it could
be a workspace member. And if it's a workspace member, there could be
sources defined in the workspace root. But we can't know without
fetching the workspace root -- and we need the workspace in order to
find the root...

Closes #10568.

crates/uv-git/src/git.rs

@@ -102,7 +102,7 @@ impl GitReference {
     }
 
     /// Converts the [`GitReference`] to a `str` that can be used as a revision.
-    pub(crate) fn as_rev(&self) -> &str {
+    pub fn as_rev(&self) -> &str {
         match self {
             Self::Tag(rev) => rev,
             Self::Branch(rev) => rev,