[FIX] stock: copy over pack_lot_ids when clicking on 'split'

Analogous to 6fde72abeacdac6e36a0c623c024252839f993ae but now instead
for the 'split' button.

Additionally this uses float_compare with the rounding set on the uom of
the operation to determine if there are any units left, which is more
correct.

opw-685908