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A file called "package/__main__.py" is executed as a script by "python -m package". See <https://docs.python.org/dev/library/__main__.html>.
I’ve came across this issue myself. You don’t even need the __main__.py file to be doing anything special, as long as the __init__.py raises an ImportError, I think. On Python 3.4 the report is even more convoluted:
/sbin/python3: Error while finding spec for 'package.__main__' (<class 'ImportError'>: No module named 'missing_module'); 'package' is a package and cannot be directly executed
I dunno what "finding spec" means, and packages _can_ be directly executed if they have a __main__ module, so at least the last bit is definitely wrong.