[FIX] sale{,_stock}: decrease qty of SOL

When decreasing the qty of a SOL, if there is already a packaging
defined, a return will be created for the difference

To reproduce the issue:
1. Create a product P
    - Storable
    - With one packaging PK for 10 x P
2. Create and confirm a SO
    - Order Lines:
        - 10 x P with PK
3. On the SO, decrease the quantity of P to 8

Error: a return is created for the difference. Instead, the existing
picking should be updated

At some point, we try to merge the stock moves (`10 x P` and `-2 x P`)
and, to do so, they must have the same `product_packaging_id`:
https://github.com/odoo/odoo/blob/08f53ffa30479cfb452ebaed61b42655eb0e6a95/addons/stock/models/stock_move.py#L808-L813

However, on the SO, when decreasing the quantity, an onchange tries to
find the best packaging. Because the new quantity is not a multiple of
PK, this one is not returned. As a result, the SO has no more packaging
and so does the new stock move (`-2 x P`). Therefore, the stock moves
can not be merged and a new picking (the return) is created.

This commit updates the onchange method so it behaves the same way as
purchase side:
https://github.com/odoo/odoo/blob/20fa7c6f0f5c9cdfae2519ea8d0ff849d7fa3b9e/addons/purchase/models/purchase.py#L1224-L1231
In this onchange, the same diff has recently be added for another reason
(see [1])

[1] 20fa7c6f

OPW-3027110

Part-of: odoo/odoo#105430