When the @six//:six library is used on a target without the
`legacy_create_init` flag disabled, the library will not be
usable due to __init__.py being empty and the six code will
be in six.py. This change forces six to occupy the __init__
name, preventing this file getting created regardless of
the `legacy_create_init` setting.

See comments on a74c43bb for
context:
https://github.com/protocolbuffers/protobuf/commit/a74c43bbd906a42b6bf70668c5d515c6f1d8d2cb