[FIX] purchase: trigger onchange when po is import.
[FIX] purchase: trigger onchange when po is import.

purpose of this commit is when we import po with po lines
system expects from user to import all those requried fields manually.

Using sale's hack 'prepare_add_missing_fields' fix this issue.

Now we can import po lines in purchase too.

Task- 2093205
closes- #41681
ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo