docs: def registerAddonModule(cls, module):
          """
          Registers a module to use to import addon subclasses from.
          
          :param      module | <str> || <module>
          """
          prop = '_{0}__addon_modules'.format(cls.__name__)
          mods = getattr(cls, prop, set())
          mods.add(module)
          setattr(cls, prop, mods)