pep517_backend in-tree build backend

The pep517_backend.hooks importable exposes callables declared by PEP 517 and PEP 660 and is integrated into pyproject.toml's [build-system].build-backend through [build-system].backend-path.

Design considerations

__init__.py is to remain empty, leaving hooks.py the only entrypoint exposing the callables. The logic is contained in private modules. This is to prevent import-time side effects.