Add `pylock.toml` to `uv pip install` and `uv pip sync` (#12992)
## Summary
We accept `pylock.toml` as a requirements file (e.g., `uv sync
pylock.toml` or `uv pip install -r pylock.toml`). When you provide a
`pylock.toml` file, we don't allow you to provide other requirements, or
constraints, etc. And you can only provide one `pylock.toml` file, not
multiple.
We might want to remove this from `uv pip install` for now, since `pip`
may end up with a different interface (whereas `uv pip sync` is already
specific to uv), and most of the arguments aren't applicable (like
`--resolution`, etc.). Regardless, it's behind `--preview` for both
commands.
