MessageBusFactory cannot access private service

[lunetics]

In the MessageBusFactory the container is accessed via get().

 foreach ($plugins as $pluginId) {
            $plugin = $container->get($pluginId);
            $plugin->attachToMessageBus($bus);
        }

The prooph_service_bus.container_plugin is marked private,thus cannot be accessed. This PR removes the "public=false" attribute. Any specific reason to keep it private?

Same for message factory plugins and router plugins

[codeliner]

oh, it worked in the past. If I remember correctly the idea was to mark them as private services because they are only useful together with the message bus which should be the only public service. But looking at the sf docs again I think we need to make them public.
/cc @prooph/symfony-team