[IMP] product: Change 'default_code' to a compute field
[IMP] product: Change 'default_code' to a compute field

Goal : Don't display the internal reference from one of the product template
as it was currently the case, because it was a related field to a one2many.
Here, we use the same behavior as for weight,volume or standard price fields.
If only 1 variant, i.e. no variant used, take the default_code from the template
If more than 1 variant, do not display anything.

dfang/odoo,ygol/odoo,hip-odoo/odoo,hip-odoo/odoo,hip-odoo/odoo,ygol/odoo,dfang/odoo,dfang/odoo,ygol/odoo,ygol/odoo,dfang/odoo,hip-odoo/odoo,dfang/odoo,dfang/odoo,ygol/odoo,hip-odoo/odoo,ygol/odoo,ygol/odoo,hip-odoo/odoo