File structure

* Add a suggested file structure and the connection to modules and import resolution.
* [src layout vs flat layout](https://packaging.python.org/en/latest/discussions/src-layout-vs-flat-layout/)
* What belongs in a module level `__init__py`, should it be empty?
* Consider if placing the tests inside the modules make sense (see e.g. https://github.com/scikit-learn/scikit-learn/tree/main/sklearn/ensemble)

Start with the linked Python Packaging User Guide discussion and the scikit-learn ensemble tree to compare src and flat layouts and test placement. Done means the course documents a recommended file structure, its module and import-resolution relationship, guidance for module-level __init__.py files, and a position on placing tests inside modules.

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