Prevent enabling guest checkout if a checkout flow is used for license purchases

This module currently prevents you from saving a product variation type form with the license trait enabled if the checkout flow associated with the variation type's correlated order item type supports guest checkout. (Phew, that's a mouthful! 😮‍💨)

Basically, you have: product variation type -> order item type -> checkout flow, and if the checkout flow supports guest checkout, ProductVariationTypeFormAlter adds an error to the element, preventing form submission.

The problem is that after you have configured the product variation type, you can still edit the checkout flow to enable guest checkout. While we might make the module more capable in the future, perhaps it would be good in the meantime to add a form validate handler to that checkout pane configuration form to prevent enabling guest checkout once the checkout flow is being used for licenses.