[FIX] purchase: correct triggers on computed fields

To be triggered on the fields that are used inside the computed field.
Was not very problematic before as the fields were not stored and recomputed
at each modification. It could be backported if needed.

Next commit will store these fields for performance reason, fixing the trigger
first is required.

Linked to #15259

addons/purchase/models/purchase.py

@@ -57,7 +57,7 @@ class PurchaseOrder(models.Model):
             else:
                 order.invoice_status = 'no'
 
-    @api.depends('order_line.invoice_lines.invoice_id.state')
+    @api.depends('order_line.invoice_lines.invoice_id')
     def _compute_invoice(self):
         for order in self:
             invoices = self.env['account.invoice']
@@ -75,7 +75,9 @@ class PurchaseOrder(models.Model):
             types = type_obj.search([('code', '=', 'incoming'), ('warehouse_id', '=', False)])
         return types[:1]
 
-    @api.depends('order_line.move_ids')
+    @api.depends('order_line.move_ids.returned_move_ids',
+                 'order_line.move_ids.state',
+                 'order_line.move_ids.picking_id')
     def _compute_picking(self):
         for order in self:
             pickings = self.env['stock.picking']