Migrations: Metatags Quick 2 per-entity values

Postponing this until after 1.0.

8.x-1.0 is out, so this is fair game again.

@drootsad: You could try upgrading Metatags Quick to Metatag 7.x-1.x first, so then you could just use Metatag-to-Metatag upgrade (see #2563649: Migrations: Metatag-D7 basic entities).

This blog post might help: https://thinktandem.io/blog/2018/07/24/writing-a-custom-drupal-8-module-...

Am going to work on this today during the Contrib Half Hour meeting.

WIP. This is an initial fixtures file. It might need Field API definitions added.

Per heddn, look at https://www.drupal.org/docs/8/api/migrate-api/generating-database-fixtur... as a way to confirm what the actual fixture needs to contain.

While looking at this today with a group of folks we realized that this will be a little more complicated to handle.

• It has to handle multiple fields.
• It has to leverage the Field API migration system instead of building a custom data migration.
• It might then need to add "_pseudo_metatags_quick_[tag]" fields in the data mapping for each item.
• The field_metatag destination will need to combine these values into an array using a custom plugin.

As such, the existing work done for Metatag-D7 isn't overly helpful with the field migration.

Putting this on the backburner for now.