[FIX] stock: split move line in extra move

_action_done() creates extra stock move when quantity_done is superior
to initial demand. This extra move should be, after confirmation, either
merge into the original stock_move, either stay extra but the move lines
still linked to the first one should be shared accordingly to the quantity
done. The fix 561b3461 adapted the code
for MRP to split the move lines but neither those cases are applied in
MRP because the extra move has no picking_id field.

condition :
  ```if merge_into_self and extra_move.picking_id:```
will therefore be false as well as
  ```if not merge_into_self:```

To be sure the two extra move management cases are complementary, this
Commit keeps make sure the conditions are mutualy exclusive.

closes odoo/odoo#40228

Opw: 2087864
X-original-commit: 4e8ac38a6d67bcd5040af0b580bfa43da7841548
Signed-off-by: William Henrotin <Whenrow@users.noreply.github.com>