Add limitations of given spec to self's.

        Args:
            spec (PackageSpec): another spec.

def add(self, spec):
        """
        Add limitations of given spec to self's.

        Args:
            spec (PackageSpec): another spec.
        """
        for limit in spec.limit_to:
            if limit not in self.limit_to:
                self.limit_to.append(limit)