Function collect_repart_definitions 

fn collect_repart_definitions(
    dry_partitions: &[RepartPartition],
    dest_dir: &Path,
    filter: impl Fn(&RepartPartition) -> bool,
) -> Result<()>

Collect the selected repart.d definitions into dest_dir, a fresh directory.

systemd < 253 does not support the --include-partitions= option, so instead we point systemd-repart at a directory containing only the definitions we want it to act on