[FIX] mrp_subcontracting_dropshipping,purchase_stock: return to supplier

When returning a dropshipped and subcontracted product to the supplier
location, the received quantity of the PO line will be incorrect

To reproduce the issue:
1. Create two storable products P_compo, P_Finished
2. Create a BoM:
    - Product: P_finished
    - Type: Subcontracting
    - Subcontractors: a subcontractor S
    - Components: 1 x P_compo
3. Create and confirm a PO:
    - Vendor: S
    - Deliver To: Dropship
    - Drop Ship Address: a partner P
    - Products: 1 x P_finished
4. Validate the receipt
5. Create a return with 1 x P_finished:
    - Update SO/PO quantities: True
    - Return Location: Partner Locations/Vendors
6. Validate the return
7. Go back to the PO

Error: The qty received is 2, it should be 0

There is currently no code to handle the return of a dropshipped product

OPW-3030895

closes odoo/odoo#105793

X-original-commit: 8276a9b5
Signed-off-by: William Henrotin (whe) <whe@odoo.com>
Signed-off-by: Adrien Widart <awt@odoo.com>