Move all code which shouldn't be exposed into `core` or `_core`

[max-models]

This will make it more clear which components of Struphy are meant to be part of the public API.

[spossann]

To be clear, we should move actual code here, not just init.py, right?

[max-models]

To be clear, we should move actual code here, not just init.py, right?

Yes exactly