calculate usage for VPC-related things

def _find_usage_networking_sgs(self):
        """calculate usage for VPC-related things"""
        logger.debug("Getting usage for EC2 VPC resources")
        sgs_per_vpc = defaultdict(int)
        rules_per_sg = defaultdict(int)
        for sg in self.resource_conn.security_groups.all():
            if sg.vpc_id is not None:
                sgs_per_vpc[sg.vpc_id] += 1
                rules_per_sg[sg.id] = len(sg.ip_permissions)
        # set usage
        for vpc_id, count in sgs_per_vpc.items():
            self.limits['Security groups per VPC']._add_current_usage(
                count,
                aws_type='AWS::EC2::VPC',
                resource_id=vpc_id,
            )
        for sg_id, count in rules_per_sg.items():
            self.limits['Rules per VPC security group']._add_current_usage(
                count,
                aws_type='AWS::EC2::SecurityGroupRule',
                resource_id=sg_id,
            )