Migrate system broken if content_translation enabled

TLDR; if you enable content_translation without enabling migrate_drupal, all migrations are broken because of plugin discovery issues.

📌 Experimental migrate_drupal_multilingual module Fixed introduced a major (critical?) bug on using the migration system with Drupal 8.6, when having content_translation enabled, but migrate_drupal disabled.

This is because d6_entity_reference_translation.yml and d7_entity_reference_translation.yml were moved to content_translation from migrate_drupal, but they use the Drupal\migrate_drupal\Plugin\migrate\EntityReferenceTranslationDeriver class that is only available if migrate_drupal is enabled.

In this probably very common setup (i.e. any new D8 site using content_translation), attempting to use the migration system will throw the following error:

In DerivativeDiscoveryDecorator.php line 218:

  Plugin (d6_entity_reference_translation) deriver "Drupal\migrate_drupal\Plugin\migrate\EntityReferenceTranslationDeriver" does not exist.

Some possible solutions:

1. Move d6_entity_reference_translation.yml and d7_entity_reference_translation.yml back to migrate_drupal - reverts part of the cleanup done in this patch, but is simple to do.
2. Move EntityReferenceTranslationDeriver to content_translation - cleaner solution, but breaks the API, as it would no longer be Drupal\migrate_drupal\Plugin\migrate\EntityReferenceTranslationDeriver and instead Drupal\content_translation\Plugin\migrate\EntityReferenceTranslationDeriver. These migrations are the only references in core to this deriver, and migrate_drupal was an experimental module until now, so maybe we're OK to break the API?