Return the number of dependencies of this package to the given node.

        Args:
            to (Package/Module): target node.

        Returns:
            int: number of dependencies.

def cardinal(self, to):
        """
        Return the number of dependencies of this package to the given node.

        Args:
            to (Package/Module): target node.

        Returns:
            int: number of dependencies.
        """
        return sum(m.cardinal(to) for m in self.submodules)