There were manual import through importlib, we have to replace them with LodelContext calls.
Actually the plugins module is in a dirty state....

- updated scripts etc
- make tests pass

Theorically all of those imports were tested by unit testing, but we've got no inssurance about that.
And even if unit tests had check for syntax errors, all pieces of code were not tested. We cannot
be sure that an import was missed or forgotten...

They have to implement a _activate() method in their __init__.py file

- renamed Plugins class to Plugin
- an instance represent a loaded plugin
- classmethod allows to preload & load plugins