[FIX] purchase_stock: tax included in multi-currency

- Set the company currency to USD, activate multi-currency
- Create a rate of 0.5 for EUR
- Create a purchase tax T:
  Percentage: 10 %
  Included in price
- Create a PO in EUR with the followind line
  Quantity: 10.0
  Price Unit: 1.0
  Tax: T
- Validate the PO, receive the product

The journal entry is composed of 2 lines:
- debit:  18.18; amount currency:  10.0
- credit: 18.18; amount currency: -10.0

This is not correct: the amount currency should not include the tax.

In order to prevent this, we use the subtotal, and divide by the
normalized quantity (yeah, because it was also failing in case of multi
UoM, as one could have expected).

opw-1958287

closes odoo/odoo#32797

Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>

addons/purchase_stock/models/stock.py

@@ -60,7 +60,13 @@ class StockMove(models.Model):
         if self.purchase_line_id:
             purchase_currency = self.purchase_line_id.currency_id
             if purchase_currency != self.company_id.currency_id:
-                purchase_price_unit = self.purchase_line_id.price_unit
+                # Do not use price_unit since we want the price tax excluded. And by the way, qty
+                # is in the UOM of the product, not the UOM of the PO line.
+                purchase_price_unit = (
+                    self.purchase_line_id.price_subtotal / self.purchase_line_id.product_uom_qty
+                    if self.purchase_line_id.product_uom_qty
+                    else self.purchase_line_id.price_unit
+                )
                 currency_move_valuation = purchase_currency.round(purchase_price_unit * abs(qty))
                 rslt['credit_line_vals']['amount_currency'] = rslt['credit_line_vals']['credit'] and -currency_move_valuation or currency_move_valuation
                 rslt['credit_line_vals']['currency_id'] = purchase_currency.id