Restrict which names are replaced

[asmeurer]

We should

• Make sure __all__ is respected
• Don't include names that shouldn't be imported from a module. For now the main thing here is other modules, or at least standard library imports.

[asmeurer]

__all__ now works and __builtins__ is excluded in dynamic imports. Not sure if we should remove other stuff, especially in the dynamic importing. Currently if you have an __init__.py without __all__ all submodules that have been imported are included in import *. However, I suppose these might be legitimately used (but the static importer doesn't attempt to add them). Stuff likeimport sys could be excluded, but perhaps it is better to just keep it as exact as possible.