Get a set of couple (name, field) of target mixedin.

def get_mixedins_by_name(target):
        """Get a set of couple (name, field) of target mixedin."""

        result = getattr(target, Mixin.__MIXEDIN_KEY__, None)

        if result is None:
            result = dict()
            setattr(target, Mixin.__MIXEDIN_KEY__, result)

        return result