[IMP] product: Use the vendor code in 'Customer Ref' if set
[IMP] product: Use the vendor code in 'Customer Ref' if set

Currently the 'partner_ref' field is used when generating purchase orders
lines. It computed like this:

Customer Ref : Vendor Product Name on the supplier info
            OR internal reference of the product
            OR product name

As we can set an alternative internal reference for the vendor,
that should be used instead of the internal reference of the product,
this commit will compute it as follows:

Customer Ref : Vendor Product Name on the supplier info
            OR vendor product code on the supplier info
            OR internal reference of the product
            OR product name

closes odoo/odoo#28573
ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo