Fix problem discovered by Greg McFarlane: when an imported module replaces its own entry in sys.module, reference count errors ensue; even if there is no reference count problem, it would be preferable for the import to yield the new thing in sys.modules anyway (if only because that's what later imports will yield). This opens the road to an official hack to implement a __getattr__ like feature for modules: stick an instance in sys.modules[__name__].