[FIX] point_of_sale: flattening only group of taxes when creating amls

Revision 89c63096cb fixed the creation
of the taxes applied to account move lines when validating a pos session

The fix though overlooked two things:
- protection against recursivity
(nothing prevents a group of taxes to have a grand-parent as one of its children)
- the fact that when converting a group of taxes into a regular computed one,
the field children_ids is never reset

Those two caveats provoked a "max recursivity error" in python within the right conditions

This commit protects the recursive process from the two issues, and there is no traceback

OPW 816163

addons/point_of_sale/models/pos_order.py

@@ -197,10 +197,14 @@ class PosOrder(models.Model):
         return InvoiceLine.sudo().create(inv_line)
 
     def _create_account_move_line(self, session=None, move=None):
-        def _flatten_tax_and_children(taxes):
+        def _flatten_tax_and_children(taxes, group_done=None):
             children = self.env['account.tax']
-            for tax in taxes:
-                children |= _flatten_tax_and_children(tax.children_tax_ids)
+            if group_done is None:
+                group_done = set()
+            for tax in taxes.filtered(lambda t: t.amount_type == 'group'):
+                if tax.id not in group_done:
+                    group_done.add(tax.id)
+                    children |= _flatten_tax_and_children(tax.children_tax_ids, group_done)
             return taxes + children
 
         # Tricky, via the workflow, we only have one id in the ids variable