Phase 1:
- Finish _setuptools_stub
- "port" build_app to something new
  (with minimal refactoring)
- Refactor for easier testing

Phase 2:
- Documentation!
- Port to modulegraph2
- Rework recipes

Phase 3:
- Write macholib2
- Port to macholib2

Extras:
- Start private github repository for
  integration testing (possibly with
  minimal variant here to make it easier
  to ask for reproducers)
- This repro will build and test with
  various libraries (PyObjC, Tkinter, Qt,
  anything requiring a recipe, ...)
- See issue about recipes for top N
  packages on PyPI, those need integration
  tests as well.
