Document that packaging doesn’t create __init__.py files (#3902).

Document that packaging doesn’t create __init__.py files (#3902).

The bug reported expected distutils to create an __init__.py file for a
project using only C extension modules.  IMO, how Python imports
packages and submodules is well documented, and it’s never suggested
that distutils might create an __init__.py file, so I’m adding this
clarification to the packaging docs but won’t backport unless other
people tell me they shared the same wrong expectation.

Thanks to Mike Hoy for his help with the patch.