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`.

从 #2786 中的 extra marker 评估工作和 #2747 中预定义的解释器设置开始。跟踪 parse_requirements 如何为 Plumbing 映射输入，然后确定如何将 py.lock TOML 转换为 JSON，并通过类似 pip.from_lock 的 API 对外提供；完成标准是能够解析 PEP 751 lockfile 并将其集成到 bzlmod 中。

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