review Python related file organization
- [x] Some important sources are somewhat hidden under Wrap/Python/src/bornagain.
- [x] Under Wrap/Python, we have files `__init__.py.in` and `lib/__init__.py`. How do they relate to each other?
- [x] In the build process, place py311 etc direcly under build; no need for intermediate `PythonPackage` directory
issue