Issue #2719735 by mikeryan: Lazy-create migration groups as needed

config/install/migrate_plus.migration_group.default.yml

-# A default group, assigned to any migrations with no explicit group.
-id: default
-label: Default
-description: A container for any migrations not explicitly assigned to a group.

migrate_plus.module

@@ -23,7 +23,20 @@ function migrate_plus_migration_plugins_alter(array &$migrations) {
     }
     $group = MigrationGroup::load($migration['migration_group']);
     if (empty($group)) {
-      continue;
+      // If the specified group does not exist, create it. Provide a little more
+      // for the 'default' group.
+      $group_properties = [];
+      $group_properties['id'] = $migration['migration_group'];
+      if ($migration['migration_group'] == 'default') {
+        $group_properties['label'] = 'Default';
+        $group_properties['description'] = 'A container for any migrations not explicitly assigned to a group.';
+      }
+      else {
+        $group_properties['label'] = $group_properties['id'];
+        $group_properties['description'] = '';
+      }
+      $group = MigrationGroup::create($group_properties);
+      $group->save();
     }
     $shared_configuration = $group->get('shared_configuration');
     if (empty($shared_configuration)) {