[FIX] stock: qty in pack operations

- Enable lot tracking on Product A
- UoM of Product A is kg, but Purchase UoM is lb(s)
- Create a PO for 200 lb(s) of Product A, confirm
- Go in the picking: 200.01 lb(s) shuold be received according to the
  pack operation, while 200.00 lb(s) are expected in the stock move.

The field `product_qty` on the sotck move contains the quantity in kg,
but already rounded from the value in lb(s). If we convert it back to
lb(s), there might obviously be a rounding error, since we do:
lb(s) -> kg -> lb(s)

To solve this, we recompute the quantity in the UoM of the product
without rounding, as a temporary computational value.

opw-705259

addons/stock/models/stock_picking.py

@@ -598,7 +598,10 @@ class Picking(models.Model):
                     continue
                 move_quants = move.reserved_quant_ids
                 picking_quants += move_quants
-                forced_qty = (move.state == 'assigned') and move.product_qty - sum([x.qty for x in move_quants]) or 0
+                forced_qty = 0.0
+                if move.state == 'assigned':
+                    qty = move.product_uom._compute_quantity(move.product_uom_qty, move.product_id.uom_id, round=False)
+                    forced_qty = qty - sum([x.qty for x in move_quants])
                 # if we used force_assign() on the move, or if the move is incoming, forced_qty > 0
                 if float_compare(forced_qty, 0, precision_rounding=move.product_id.uom_id.rounding) > 0:
                     if forced_qties.get(move.product_id):