d6_entity_reference_translation and d7_entity_reference_translation depend on migrate_drupal

In the aftermath of splitting out multi-lingual migrations, it was discovered in #2953360: Experimental migrate_drupal_multilingual module that d6_entity_reference_translation and d7_entity_reference_translation have hidden dependencies on migrate_drupal. They will not be discovered unless migrate_drupal is also enabled.

To have these migrations function and be discovered, simply enable migrate_drupal.

Before (migrations resided directly in migrate_drupal)
d6_entity_reference_translation
d7_entity_reference_translation

After (migrations reside in content_translation)
d6_entity_reference_translation
d7_entity_reference_translation