cybersource_commerce/checkout depends on an invalid library

I took a first pass at this, but realized that doing this in an alter hook won't work. I think there's actually a design issue with embedding JS in the module.

CyberSource provides two different URLs for the Flex library - one for production, and one for test. The module allows for each configured gateway to have it's own settings for production or test mode. However, the JS defines the same object ("Flex"), so if you try to embed both libraries at the same time you'll get an error. And if you only want to embed one, you can't know which payment gateway is being used when using `#attached`.

I think the prod / test mode should be moved to a global setting instead of being set on each payment gateway configuration entity. However, this could be considered a breaking change because there's no way to reconcile conflicting payment gateway settings in an update hook without intervention. That could be done in a new major release?

For 1.x, another option would be to remove the library dependency and document it with a comment. Then, we add something like this in checkout.js:

if (typeof Flex === 'undefined') {
  console.error('Flex.js is not loaded. You must attach it manually as it cannot be included as a Drupal Library dependency.');
  return;
}

One thing I don't really get: why would you need to manually attach the library to the form yourself?

This was in inherited code that I wasn't involved in, and I don't have information as to why it was done this way. Since filing this issue, we ended up deciding to do a major refactor, simplification, and upstreaming of code leading to #3221909: Add support for creating stored payment methods via the Flex Microform, #3548932: Show error messages when card validation fails, and #3549621: Support ACH / Check payments. Hopefully no one else goes down that path in the future!

Yes, lets, and thank you!