Since DefaultPartitionGrouper is only for the purpose of the internal StreamsPartitionAssignor it would make sense to have it in the org.apache.kafka.streams.processor.internals package.
I would also vote to move PartitionGrouper.
KAFKA-8927 we deprecated the `PartitionGrouper` interface in 2.4 release – this allows us to remove the public interface and its corresponding config in the next major release (ie, 3.0.0). `DefaultPartitionGrouper` was implicitly deprecated via KAFKA-8927.
Hence, we can move the interface as well as the default implementation into an internal package (or maybe just remove the interface completely as there are no plans to support multiple implementations atm).