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`.

Start with the extra marker evaluation work in #2786 and the predefined interpreter setting from #2747. Trace how parse_requirements maps input for plumbing, then determine how py.lock TOML should become JSON and be exposed through an API such as pip.from_lock; done means the PEP 751 lockfile can be parsed and integrated into bzlmod.

Written by the indexing model from the issue text.