Support PEP751 lockfile format in bzlmod

After `pip lock` lands, this might become a better `requirements.txt` alternative.

References:
* pip (25.1) will export a lockfile - ETA 2 weeks https://github.com/pypa/pip/pull/13213
* packaging has landed the marker resolution (unlocks PDM, poetry, hatch) https://github.com/pypa/packaging/issues/885
* uv is tracking it as an export format https://github.com/astral-sh/uv/issues/12584
* poetry still thinking about it https://github.com/orgs/python-poetry/discussions/10322
* pip-tools keen https://github.com/jazzband/pip-tools/issues/2124

This most likely depends on #2786.

Making this work needs:
- [ ] Extra marker evaluation support (#2786).
- [ ] Parse toml to JSON using some predefined interpreter (set with #2747 `.defaults`).
- [ ] Map to the output like what `parse_requirements` is returning for easy plumbing.
- [ ] Call the API for parsing `py.lock` something like `pip.from_lock`.

#2786 の extra marker 評価作業と、#2747 の事前定義されたインタープリター設定から始めます。parse_requirements が Plumbing 用に入力をどのようにマッピングするかを追跡し、次に py.lock TOML を JSON に変換して pip.from_lock のような API を通じて公開する方法を決定します。完了条件は、PEP 751 lockfile をパースして bzlmod に統合できることです。

索引モデルが issue の本文から書いたものです。