Perhaps a suitable outcome here would be to just add an unconditional RuntimeWarning when the target of "-m" is already in sys.modules after the parent package import, along the lines of:

"RuntimeWarning: '<module>' already in sys.modules prior to '__main__' execution; this may result in unpredictable behaviour"

Then if folks get the "ValueError: package.module.__spec__ is not set" message, that would be an example of the noted "unpredictable behaviour", and the suggested resolution would be the path Wolfgang took: find a way to eliminate the warning when executing that particular module as __main__.