Add check for unused module-level private and protected functions

[socketpair]

test.py:

def _unused():
    pass
def __unused():
    pass
class _class():
    pass
_some_constant = 42

They should not be imported from anywhere, so if they are not used in the same module - it's definitely a bug.

This file should generate errors about unused private/protected module-level members. Just like private methods #4504