Ah, __main__, thou art such a special beast.

Here's my current plan:

1. __main__ will be initialised with __loader__ set to BuiltinImporter. This covers cases like the interactive prompt or execution of stdin where there is no source code available for __main__

2. When executing a script (either directly or via runpy), __main__.__loader__ will be *changed* at a later point to refer to the appropriate kind of loader object.