Support PEP751 lockfile format in bzlmod

[aignas]

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

References:

• pip (25.1) will export a lockfile - ETA 2 weeks PEP 751 experimental pip lock command pypa/pip#13213
• packaging has landed the marker resolution (unlocks PDM, poetry, hatch) Support PEP 751 in markers pypa/packaging#885
• uv is tracking it as an export format Add support for PEP 751 lockfiles astral-sh/uv#12584
• poetry still thinking about it https://github.com/orgs/python-poetry/discussions/10322
• pip-tools keen Implement PEP 751 "A file format to list Python dependencies for installation reproducibility" (a.k.a. lock files) jazzband/pip-tools#2124

This most likely depends on #2786.

Making this work needs:

• Extra marker evaluation support (Support extras and dependency_groups in the PEP508 env marker evaluation #2786).
• Parse toml to JSON using some predefined interpreter (set with Improve the pip.parse API to allow for incremental building of the configuration #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.