Asterisk imports in `__init__.py` submodules?

[Icemole]

In #643 @albertz noticed that in the submodule's __init__.py files we imported everything from the submodule itself with an asterisk import. For instance:

1. AM module
2. Audio module
3. LM module
4. RETURNN module

Do we want to allow this or not? I think using asterisk imports in __init__.py has upsides (more aesthetic/sensible grouping of jobs, especially when boundaries between py files are not too well defined) and downsides (many of these imports are not need at manager loading time, so the final loading time is increased).

[michelwi]

It depends.. If the submodules have a defined __all__ that contains only the necessary stuff, then we have less pollution.

One additional upside of importing all Jobs from the submodules is, that we can refactor the submodules without breaking the existing imports.

[albertz]

One additional downside is that this decision cannot be undone. Once you have it all imported, removing that would likely break some existing code.