When using AVCO and subcontracting without any additional cost, the
valuation of the finished product is incorrect

To reproduce the issue:
1. Create a Product Category PC:
    - Costing Method: AVCO
2. Create two products P_compo and P_finished:
    - Both:
        - Type: Storable
        - Category: PC
    - P_compo:
        - Cost: 10
        - Routes: Resupply Subcontractor
3. Update P_compo's quantity: 2
4. Create a Bill of Materials BO:
    - Product: P_finished
    - Type: Subcontracting
    - Subcontractor: a partner P
    - Component: 1 x P_compo
5. In Inventory, create a planned Receipt R:
    - From: P
    - Operations: 1 x P_finished
6. Mark R as To Do
7. Process the delivery of P_compo
8. Validate R
9. Repeat steps 5-8
10. Open the Inventory Valuation

Error: There are two valuation lines for P_finished: one line has a
value of $10, which is correct (this is the cost of the component).
However, the value of the second line is $20, it should be $10 too.

Here are a part of the values used to generate the related MO:
https://github.com/odoo/odoo/blob/2d12fb8fb94c0f2acade7222cfedbec34114a8e9/addons/mrp_subcontracting_account/models/stock_picking.py#L21-L25
In the above case, an extra cost is defined on the MO and is based on
the unit price of the subcontracted SM. However, `_get_price_unit` will
return an incorrect value:
https://github.com/odoo/odoo/blob/251be6b943ea8c3f274bb0863d0af3f7c6b8d10d/addons/stock_account/models/stock_move.py#L39
After step 8, the standard price of P_finished is $10. Also, in the
above case, there isn't any subcontracting cost, so there isn't any unit
price defined on the SM. Therefore, `_get_price_unit` returns the
standard price of P_finished ($10). As a result, when computing the
value of the finished stock move:
https://github.com/odoo/odoo/blob/4fc2ec31861d4602357f130066ec3507b96d8dc8/addons/mrp_account/models/mrp_production.py#L39-L42


It uses the extra cost of the MO + the component cost. This explains
where the $20 come from.

OPW-2641339

closes odoo/odoo#80373

Signed-off-by: William Henrotin (whe) <whe@odoo.com>