Change relative path behaviors for `requirements` files

[zanieb]

One part of this is

• PROJECT_ROOT in requirements uses the working directory #3271

Similarly, relative paths are relative to the working directory, not the file.

An option...

• ../<path> to be relative to the file
• ${PWD}/<path> to be relative to the cwd
• ${PROJECT_ROOT}/<path> to be relative to the nearest directory with a pyproject.toml, or, if not found, the file
• ${WORKSPACE_ROOT}/<path> to be relative to the uv workspace root, or, if not found, the file

There are a couple problems here:

• We need consensus on what it should be
• We need a way to roll it out
• We need a way to deal with pip having different behavior