Fixed a couple of long-hidden bugs (amazing what you find when you

Fixed a couple of long-hidden bugs (amazing what you find when you
attempt to verify the bold assertions in the documentation):
  * entries for the "root package" in 'package_dir' didn't work --
    fixed by improving the fall-through code in 'get_package_dir()'
  * __init__.py files weren't installed when modules-in-packages
    were listed individually (ie. in 'py_modules' in the setup script);
    fixed by making 'check_package()' return the name of the __init__
    file if it exists, and making 'find_modules()' add an entry to
    the module list for __init__ if applicable