* Use submodule imports in aqt
* Use submodule imports in pylib
* More submodule imports in pylib
These required removing some direct imports to get rid of import cycles.
The enum changes should work on PyQt 5.x, and are required in PyQt 6.x.
They are not supported by the PyQt5 typings however, so we need to run
our tests with PyQt6.
Like the previous change, avoid exposing the protobuf as a public API
for now. It requires more thought, and is probably better done with
either extra helper accessors like decks.name(), or via a native class.
