The same result can be achieved with a computed field.
Moreover, we have one already in odoo core. Originally it was a related
field, but we can change it to a computed and add our extra logic there.
Notes:
* `_get_product2bom` renamed to `_bom_find`
* Test case `test_manufacturing_order_2`:
* No longer check for "Add some materials to consume before marking this MO as to do.",
as it's already supported in core since https://github.com/odoo/odoo/commit/bf5e1debf
