Convert MigrateSource plugin discovery to attributes

Closes #3421014

core/lib/Drupal/Core/Plugin/Discovery/AttributeDiscoveryWithAnnotations.php

@@ -114,7 +114,7 @@ protected function parseClass(string $class, \SplFileInfo $fileinfo): array {
    * @see \Drupal\Component\Annotation\Plugin\Discovery\AnnotatedClassDiscovery::prepareAnnotationDefinition()
    * @see \Drupal\Core\Plugin\Discovery\AnnotatedClassDiscovery::prepareAnnotationDefinition()
    */
-  private function prepareAnnotationDefinition(AnnotationInterface $annotation, string $class): void {
+  protected function prepareAnnotationDefinition(AnnotationInterface $annotation, string $class): void {
     $annotation->setClass($class);
     if (!$annotation->getProvider()) {
       $annotation->setProvider($this->getProviderFromNamespace($class));
@@ -133,7 +133,7 @@ private function prepareAnnotationDefinition(AnnotationInterface $annotation, st
    * @see \Drupal\Component\Annotation\Plugin\Discovery\AnnotatedClassDiscovery::getAnnotationReader()
    * @see \Drupal\Core\Plugin\Discovery\AnnotatedClassDiscovery::getAnnotationReader()
    */
-  private function getAnnotationReader() : SimpleAnnotationReader {
+  protected function getAnnotationReader() : SimpleAnnotationReader {
     if (!isset($this->annotationReader)) {
       $this->annotationReader = new SimpleAnnotationReader();