sale_purchase_force_vendor: Be able to force a vendor on other MTO-like routes

[marielejeune]

The module sale_purchase_force_vendor allows the user to force a vendor (supplier) on a sale order line. If using the default MTO route, the purchase order will be created targeting the vendor specified on the SOL.

However, a MTO route is just a route that contains a rule "Pull from Partners/Customers" whose supply method is "trigger another rule", to be combined with another rule/route that fulfills the source location (stock).

On more complex warehouses one may want to define MTO-like routes that combine several "pull from" rules and a "buy" rule. See a concrete example below:

The actual code in sale_purchase_force_vendor doesn't propagate the vendor through the different rules of a single route. This is why it works only with the Odoo default MTO route.

An enhancement would like to propagate the related SOL to be able to retrieve the forced vendor when triggering the rule buying the product.

See related PR #2575 for an enhancement proposition.

[rousseldenis]

@OCA/procurement-purchase-maintainers