Help message are no longer searched in `__main__` modules and in modules that are not packages.

Closes #2.

This adds a (minor) backward incompatibility, since some help messages are no longer searched in the same module they were searched before (or a no longer searched at all). However, this was necessary because it fixes a bad design. For instance, searching a help message in the docstring of a `__main__` package means importing it to access the docstring, and (maybe) importing it again to run it. This is discouraged, and displayed the warning (described in #2).