Deprecate drupal_classloader_register() & system_register()

+++ b/core/includes/module.inc
@@ -72,7 +72,7 @@ function system_list_reset() {
  */
 function system_register($type, $name, $uri) {
   drupal_get_filename($type, $name, $uri);
-  drupal_classloader_register($name, dirname($uri));
+  \Drupal::service('class_loader')->addPsr4('Drupal\\' . $name . '\\', \Drupal::root() . '/' . dirname($uri) . '/src');

system_register() seems unused in core, instead of updating the usage there we could also just not touch it and instead deprecate it too? Done that in some cases, but might be easier to update it.

+++ b/core/includes/bootstrap.inc
@@ -790,8 +790,21 @@ function drupal_get_profile() {
+ * @deprecated in Drupal 8.7.0, will be removed before Drupal 9.0.0. Use the
...
+  @trigger_error('drupal_classloader_register() is deprecated in Drupal 8.7.0 and will be removed before Drupal 9.0.0. Use the method ::addPsr4() of the class_loader service to register the namespace. See https://www.drupal.org/node/3035275.', E_USER_DEPRECATED);

+++ b/core/includes/module.inc
@@ -69,10 +69,18 @@ function system_list_reset() {
+ * @deprecated in Drupal 8.7.0 and will be removed before Drupal 9.0.0. There is
...
+  @trigger_error('system_register() is deprecated in Drupal 8.7.0 and will be removed before Drupal 9.0.0. There is no replacement for this function. To achieve the same functionality use this snippet: $path = \Drupal::service("extension.list.$type")->getPath($name); ' . "\\Drupal::service('class_loader')->addPsr4('Drupal\\\\' . \$name . '\\\\', \\Drupal::root() . '/' . \$path . '/src'); See https://www.drupal.org/node/3035275.", E_USER_DEPRECATED);

+++ b/core/tests/Drupal/Tests/Core/ClassLoader/ClassLoaderTest.php
@@ -0,0 +1,46 @@
+   * @expectedDeprecation drupal_classloader_register() is deprecated in Drupal 8.7.0 and will be removed before Drupal 9.0.0. Use the method ::addPsr4() of the class_loader service to register the namespace. See https://www.drupal.org/node/3035275.
...
+   * @expectedDeprecation system_register() is deprecated in Drupal 8.7.0 and will be removed before Drupal 9.0.0. There is no replacement for this function. To achieve the same functionality use this snippet: $path = \Drupal::service("extension.list.$type")->getPath($name); \Drupal::service('class_loader')->addPsr4('Drupal\\' . $name . '\\', \Drupal::root() . '/' . $path . '/src'); See https://www.drupal.org/node/3035275.

Deprecated in 8.8.x now but LGTM otherwise.

Addressed #6

Looks ready

Committed 8165415 and pushed to 8.8.x. Thanks!

#9, Updated CR branch.

publish the cr