[FIX] purchase: real time average multicurrency valuation

Do a PO in a foreign currency
Invoice it in the same foreign currency
Validate the invoice

Before this commit, The account move of the invoice
contained a price difference line
which is wrong, since the price has not changed in any ways

After this commit, the move has only two lines
one in the stock input, one in the payable

Also, the case where the price does change
due to a change in currency rate is handled

OPW 1955315

closes odoo/odoo#32180

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