Prepare the master working set.

def _build_master(cls):
        """
        Prepare the master working set.
        """
        ws = cls()
        try:
            from __main__ import __requires__
        except ImportError:
            # The main program does not list any requirements
            return ws

        # ensure the requirements are met
        try:
            ws.require(__requires__)
        except VersionConflict:
            return cls._build_from_requirements(__requires__)

        return ws