This does not yet fix `derive(Action)`, but will conflict with a lot so
wanted to merge quickly.

Although automatically deriving the namespace worked in many situations,
it was
unclear what to do with nested modules. Vim wanted all actions to be
registered
under vim, while collab_ui wanted one namespace per action.

It seems better to make the implicit explicit, and give the flexibility
to hide
implementation details from the end-users.

Release Notes:

- N/A