Eventually implement support for the PEP 735 "dependency groups"

What's the problem this feature will solve?

It's just a new source of deps and an extension to the first round of the pyproject.toml runtime deps resolution.

Describe the solution you'd like

$sbj. If somebody's interested in making a PoC, it's probably fine to start drafting something already. Need to wait for the PEP acceptance to actually merge it in, though.

Alternative Solutions

... not doing this?

Additional context

Refs:

• https://peps.python.org/pep-0735/
• https://discuss.python.org/t/pep-735-dependency-groups-in-pyproject-toml/39233/237
• POC for pip: https://github.com/pypa/pip/compare/main...sirosen:pip:pep-735-support