Issue #3228311: Use a standard migration derivatives ID delimiter

Closes #3228311

src/MigrateUpgradeDrushRunner.php

@@ -422,7 +422,7 @@ class MigrateUpgradeDrushRunner {
    *   The ID modified to serve as a configuration entity ID.
    */
   protected function modifyId($id) {
-    return $this->options['migration-prefix'] . str_replace(':', '_', $id);
+    return $this->options['migration-prefix'] . str_replace(':', '__', $id);
   }
 
   /**