Multibind should respect the `@singleton` decorator

A failing test case:

def test_multibind_respects_the_singleton_decorator() -> None:
    @singleton
    class SingletonPlugin(Plugin):
        pass

    def configure(binder: Binder) -> None:
        binder.bind(Plugin, to=SingletonPlugin)
        binder.multibind(List[Plugin], to=SingletonPlugin)
        binder.multibind(List[Plugin], to=[SingletonPlugin])
        binder.multibind(Dict[str, Plugin], to={'singleton1': SingletonPlugin, 'singleton2': SingletonPlugin})

    injector = Injector([configure])
    plugin_singleton = injector.get(Plugin)
    plugins_list = injector.get(List[Plugin])
    plugins_dict = injector.get(Dict[str, Plugin])

    assert plugin_singleton is injector.get(Plugin)
    assert plugins_list[0] is plugins_list[1]  # Fails
    assert plugins_dict['singleton1'] is plugins_dict['singleton2']  # Fails

Start by locating and running the provided test named test_multibind_respects_the_singleton_decorator. Trace the Injector behavior for the direct Plugin binding and the list and dictionary multibind cases. Done means the repeated SingletonPlugin entries resolve to the same instance in both collections and the assertions pass.