Prevent `uv export` from overwriting `pyproject.toml` (#16745)
Currently, it's possible for `uv export` to overwrite someones
`pyproject.toml`. This diff simply rejects project files passed in with
`-o`, so we avoid doing that.
---------
Co-authored-by: konstin <konstin@mailbox.org>
Co-authored-by: Charlie Marsh <charlie.r.marsh@gmail.com>
