Retrieve dependencies from files referenced with -r <requirements-file>

[mkniewallner]

Right now we ignore nested requirements files referenced with -r <requirements-file>, for instance in:

-r requirements-typing.txt

pytest==8.3.4
ruff==0.8.4

This is not too problematic as it's possible to pass the file manually with --requirements-file or --dev-requirements-file on the CLI, but it could be nice to do that without the user having to specify the additional requirements file.

Note that the format without whitespace between -r and the file name is also valid, so we also need to handle -rrequirements.typing.txt for instance.