[FIX] purchase(_stock): make orderpoint qtys depend on POs

Before this commit, `stock.warehouse.orderpoint` `_compute_qty` didn't
depend on `purchase.order.line` (POL). This means when a relevant POL
was edited/cancelled, then the `qty_forecast` wouldn't correctly update.

To reproduce:
- Create a product w/ a vendor and buy route active,
- Go to replenishment report, create a reordering rule, and hit
  order for product (RFQ should be created)
- Cancel the RFQ and go back to replenishment report

The product should reappear in the list view, but does not because it's
`qty_forecast` has not updated and therefore has not updated
corresponding `qty_to_order` value.

Note string rename of `purchase_line_warn` 'Purchase Order Line' =>
'Purchase Order Line Warning' is due to repeat label use issue. It is
better to rename the poorly labeled `purchase_line_warn` than to give a
hacky label for new One2many field.

Task: 2285912
Manual forward port of: odoo/odoo#56580 due to migration test error

closes odoo/odoo#56625

Closes: odoo/odoo#56625
Signed-off-by: Arnold Moyaux <amoyaux@users.noreply.github.com>

addons/purchase/models/product.py

@@ -22,7 +22,7 @@ class ProductTemplate(models.Model):
         ('receive', 'On received quantities'),
     ], string="Control Policy", help="On ordered quantities: Control bills based on ordered quantities.\n"
         "On received quantities: Control bills based on received quantities.", default="receive")
-    purchase_line_warn = fields.Selection(WARNING_MESSAGE, 'Purchase Order Line', help=WARNING_HELP, required=True, default="no-message")
+    purchase_line_warn = fields.Selection(WARNING_MESSAGE, 'Purchase Order Line Warning', help=WARNING_HELP, required=True, default="no-message")
     purchase_line_warn_msg = fields.Text('Message for Purchase Order Line')
 
     def _compute_purchased_product_qty(self):

addons/purchase_stock/models/product.py

@@ -23,6 +23,8 @@ class ProductProduct(models.Model):
     _name = 'product.product'
     _inherit = 'product.product'
 
+    purchase_order_line_ids = fields.One2many('purchase.order.line', 'product_id', help='Technical: used to compute quantities.')
+
     def _get_quantity_in_progress(self, location_ids=False, warehouse_ids=False):
         if not location_ids:
             location_ids = []

addons/purchase_stock/models/stock.py

@@ -183,6 +183,11 @@ class Orderpoint(models.Model):
         'product.supplierinfo', string='Vendor', check_company=True,
         domain="['|', ('product_id', '=', product_id), '&', ('product_id', '=', False), ('product_tmpl_id', '=', product_tmpl_id)]")
 
+    @api.depends('product_id.purchase_order_line_ids', 'product_id.purchase_order_line_ids.state')
+    def _compute_qty(self):
+        """ Extend to add more depends values """
+        return super()._compute_qty()
+
     @api.depends('route_id')
     def _compute_show_suppplier(self):
         buy_route = []