closes #11440 ?

This is more of a proof of concept, I can try improving it and adding tests if this approach is deemed to be ok.

Since actual --only-deps option opens up a lot of rabbit holes (when installing anything other than .) and installing only dependencies of some remote package for example doesn't seem to be useful to me, this PR just handles dependencies from pyproject.toml in -r, e.g. pip install -r pyproject.toml.

I guess this is technically a breaking change, but I don't know what kind of person keeps their requirements in non-toml format in a file named pyproject.toml. -r pyproject.toml is also apparently what uv already does.

Only main dependencies (the ones in project.dependencies) are handled (for now?). Trying to do install -r pyproject.toml on pyproject with dynamic dependencies results in an error.

As is, I believe this PR is enough to address at least my use case for this: it allows caching dependency layers in docker easily, without additional tools.