Registers a module to use to import addon subclasses from.
        
        :param      module | <str> || <module>

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)