Returns the modulename.executable string that uniquely identifies
        the executable that this dependency points to.

def external_name(self):
        """Returns the modulename.executable string that uniquely identifies
        the executable that this dependency points to."""
        target = self.target
        if target is not None:
            return "{}.{}".format(target.name.lower(), self.name)
        else:
            return "{}.{}".format(self.module.name.lower(), self.name)