[FIX] sale_timesheet_margin: prevent cost override on product service

Current behavior :
a) For a service product with Invoicing Policy set to Prepaid/Fixed Price, when we create a Sales Order with this product and change his Cost with a custom value and confirm the SO, the product's cost is overridden by the product's default cost.
b) If we create a service, with Invoicing Policy set to Prepaid/Fixed Price, its cost will be 0 even if we update it in the SO

Steps to reproduce :
1. Install Sales and enable Margins in app's settings
2. Install Timesheets
3. Get a service type product with Invoicing Policy set to Prepaid/Fixed Price
4. Create a Sales Order
5. Add the service product from step 2
6. Change his cost
7. Confirm the SO

for b) , similar steps just create a new product (on the fly for example) and change the product type to Service then save.

Reason :
a) After adding this line (https://github.com/odoo/odoo/blob/15.0/addons/sale_timesheet_margin/models/sale_order_line.py#L13 - commit 8bbbc69e40) the purchase price was recalculated when it should be, which resulted in an override with the standard product price
b) When a service with Invoicing Policy set to Prepaid/Fixed Price is created, it will be considered as linked to timesheets and the cost will be 0, waiting to be recalculated with the linked timesheets (which won't happen because no linked timesheet will be created)

Details :
For b), I added a condition on the service policy because, according to this declaration https://github.com/odoo/odoo/blob/f81063382d551261801288b66e4d75303a05cbb4/addons/sale_timesheet/models/product.py#L11-L12, a service whose invoicing policy is set to Prepaid/Fixed Price will have its delivery method set to 'timesheet' and will therefore be considered as linked to a timesheet (which is false in this case) which will result in a cost equal to 0

OPW-2689756

closes odoo/odoo#80537

X-original-commit: e260a97392e698c88741be713a6b8cc2dc3c74a1
Signed-off-by: Laurent Stukkens (ltu) <ltu@odoo.com>

addons/sale_timesheet_margin/models/sale_order_line.py

@@ -8,9 +8,10 @@ class SaleOrderLine(models.Model):
     @api.depends('analytic_line_ids.amount', 'qty_delivered_method')
     def _compute_purchase_price(self):
         timesheet_sols = self.filtered(
-            lambda sol: sol.qty_delivered_method == 'timesheet' and not sol.product_id.standard_price
+            lambda sol: sol.qty_delivered_method == 'timesheet' and not sol.product_id.standard_price and not sol.product_id.service_policy == 'ordered_timesheet'
         )
-        super(SaleOrderLine, self - timesheet_sols)._compute_purchase_price()
+        already_computed_service = self.filtered(lambda sol: sol.create_date is not False and sol.product_id.service_policy == 'ordered_timesheet')
+        super(SaleOrderLine, self - timesheet_sols - already_computed_service)._compute_purchase_price()
         if timesheet_sols:
             group_amount = self.env['account.analytic.line'].read_group(
                 [('so_line', 'in', timesheet_sols.ids), ('project_id', '!=', False)],