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.