Add a filter to the list.

        Args:
            package_filter (`PackageFilter`): Filter to add.

def add_filter(self, package_filter):
        """Add a filter to the list.

        Args:
            package_filter (`PackageFilter`): Filter to add.
        """
        filters = self.filters + [package_filter]
        self.filters = sorted(filters, key=lambda x: x.cost)