Turbopack: ensure max merge segments is respected accros families (#82223)
### What?
We try to limit the maximum number of merge jobs with the `max_merge_segment_count` option. But as we run `get_merge_segments` for every key family it only limits the merge jobs per family. This results in much more merge jobs then configured.
This change applied the limits across families. Lower number families will be compacted first, which is a new behavior, so maybe we want to reorder the key families to improve the order (see enum KeySpace)