feat: Check for __all__ presence in __init__.py files

[AcquaDiGiorgio]

See #426

I tried something more sofisticated in python using importlib to check the variables in the module, both instantiated and imported, but there were multiple situations that were extremely difficult to check.

We also might want to set a common way of defining this variable, using either tuples or lists:

• diracx/routers/health/__init__.py: __all__ = ["router"]
• diracx/db/os/__init__.py __all__ = ("JobParametersDB",)

[read-the-docs-community]

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