[FIX] stock: preserve proposed serial/lot numbers after put in pack

When a stock.pack.operation gets created by a stock.picking, it will
have its pack_lot_ids set. This way the user has an overview of what
lots are available and how many of each are still left.

When splitting the stock.pack.operation these where lost. The still
unpacked stock.pack.operation would always have an empty pack_lot_ids
field. The reason for this is that those records where reassigned to the
newly created and packed stock.pack.operation.

This resolves the issue by copying pack_lot_ids that are still
available (qty_todo > 0) and assigning them to the original
stock.pack.operation.

opw-685908