[FIX] purchase: anglosaxon accounting: fix wrong computation of prod qty when multiple stock move

Have a PO with a product in automated valorization, and average as method of valorization
Validate and receive *partially* (create backorder)
invoice with a different price than the PO
Receive the backorder
Invoice the rest, also with a different price than the PO

Before this commit, wrong amounts were written in the account.move corresponding to the invoice.
In particular, the payable account's amount was erroneous because it is computed to balance the move
As a result, the bill showed an amount due different from its total, even though nothing was reconciled with it.

After this commit, the payable entry and hence the amount due on the invoice are correct

OPW 1967289

closes odoo/odoo#32920

Signed-off-by: Lucas Perais (lpe) <lpe@odoo.com>