repart: try harder to find verity-sig partitions for CopyBlocks=auto

repart: try harder to find verity-sig partitions for CopyBlocks=auto

verity-sig partitions are not kernel concepts, hence dm-verity won't
link them for us from the slaves/ subdir in sysfs. Hence let's instead
look up the partition via udev's database.

Hence: when we search for the data+verity+verity-sig partitions then
search for the first two as usual, but search for the latter by looking
up the udev props on the first two, and then following the paths
provided therein.

Fixes: #34835