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Well, I want backwards-compatibility *now*, not forever.
And no, it is not an obvious API as you are asking for what loaders are supposed to do; load a module, which is why the one-liner I gave you works today. Finder simply find a loader that can load something that they are asked to discover. Loader then load something that they are told to load, whether it was found through a finder for sys.path or explicitly pointed at something.
Another option is to have SourceFileLoader.load_module() take no argument since its constructor already has everything needed to load a module passed in which simplifies the API a little bit. But that would be a non-standard broadening of the loader API and I don't know if people will like that.