[FIX] stock: return packages

Steps to reproduce:
- Deliver an SN-tracked product with the Destination Package set (put in pack button also sets this).
- Return that product without setting the Source Package (you can also click "put in pack" which will put the product in yet another pack).
- This already results in two lines with the same SN in the same location, one with +1.00 and one with -1.00 quantity.
- Deliver that product again and put in pack.
- Now there's three lines with the same SN in the same location, two with +1.00 and one with -1.00 quantity.

Bug:
source Package isn't set bydefault when confirming the move_line
we update the stock quantity for that lot_id and Package set to False
the existing quantity has a Package set so it is filtered out and a new
negative quantity is created

Fix:
set the source Package on the return

opw-3179388

closes odoo/odoo#126308

Signed-off-by: William Henrotin (whe) <whe@odoo.com>