migrate_drupal highjacks the Migration and MigrationStorage classes

Problem/Motivation

In discussing the test module for a CSV source plugin for migrations, which was breaking for him but not for me, @phenaproxima and I realized it was breaking on Drupal source-specific stuff (applyCckFieldProcessors()) in Drupal\migrate_drupal\MigrationStorage because he had migrate_drupal enabled but I didn't. The problem:

function migrate_drupal_entity_type_alter(array &$entity_types) {
  /** @var \Drupal\Core\Config\Entity\ConfigEntityType[] $entity_types */
  $entity_types['migration']
    ->setClass('Drupal\migrate_drupal\Entity\Migration')
    ->setHandlerClass('storage', 'Drupal\migrate_drupal\MigrationStorage');
}

I.e., when migrate_drupal is enabled, it injects its own Migration and MigrationStorage classes in place of those in Drupal\migrate. And those classes, which provide functionality specific to migrate_drupal, will also be used for any non-migrate_drupal migrations. Not good. hook_entity_type_alter() should only be used when the type needs to be altered globally - i.e., when adding functionality that applies to all instances of the type. It is not an appropriate mechanism for a module to add functionality specific to its own needs.

Proposed resolution

Not quite sure at the moment...

Remaining tasks

TBD

User interface changes

N/A

API changes

TBD