Devendoring `pyproject_metadata`

[LecrisUT]

This is a tracker issue for devendoring pyproject_metadata. Currently scikit-build-core uses the latest versions of pyproject_metadata and it can affect the checksums of the tests, so it makes sense to keep it vendored for now until everything is stable. According to the last discussion a plan is to implement most of the interface needed within packaging. As soon as any course of action is taken we should update this tracker.