This PR contains two changes: The companion PR to
https://github.com/astral-sh/packse/pull/277, which moderately
simplifies the uv side, and switching to pylock.toml for packse as
dogfooding. These changes can be applied independent from each other.
Since all files, including the vendored build dependencies, are now on
GitHub Pages under the same root, we only need a packse index root URL.
konsti