Consider moving some routes and controllers out of system module

From @godotislate in the other issue:

This is a great idea. Discovery loops through the container namespaces, which include these in Drupal\Core:

   'Drupal\\Core\\Action' => 'core/lib/Drupal/Core/Action',
   'Drupal\\Core\\Block' => 'core/lib/Drupal/Core/Block',
   'Drupal\\Core\\Config' => 'core/lib/Drupal/Core/Config',
   'Drupal\\Core\\Datetime' => 'core/lib/Drupal/Core/Datetime',
   'Drupal\\Core\\DefaultContent' => 'core/lib/Drupal/Core/DefaultContent',
   'Drupal\\Core\\Entity' => 'core/lib/Drupal/Core/Entity',
   'Drupal\\Core\\Extension' => 'core/lib/Drupal/Core/Extension',
   'Drupal\\Core\\Field' => 'core/lib/Drupal/Core/Field',
   'Drupal\\Core\\Mail' => 'core/lib/Drupal/Core/Mail',
   'Drupal\\Core\\Menu' => 'core/lib/Drupal/Core/Menu',
   'Drupal\\Core\\Path' => 'core/lib/Drupal/Core/Path',
   'Drupal\\Core\\Plugin' => 'core/lib/Drupal/Core/Plugin',
   'Drupal\\Core\\Recipe' => 'core/lib/Drupal/Core/Recipe',
   'Drupal\\Core\\Render' => 'core/lib/Drupal/Core/Render',
   'Drupal\\Core\\TempStore' => 'core/lib/Drupal/Core/TempStore',
   'Drupal\\Core\\Theme' => 'core/lib/Drupal/Core/Theme',
   'Drupal\\Core\\TypedData' => 'core/lib/Drupal/Core/TypedData',
   'Drupal\\Core\\Validation' => 'core/lib/Drupal/Core/Validation',
   'Drupal\\Core\\Config\\Action' => 'core/lib/Drupal/Core/Config/Action',

I tested moving EntityAutocompleteController to Drupal\Core\Entity\Controller locally, and it seems to work correctly. I'm not sure if we should do that in this issue though, because maybe we'd want to move and update some corresponding tests as well. We can probably do a separate issue to evaluate what routes in system should be moved to Core and where.

I think this only works for EntityAutocompleteController and TimezoneController. For the others we need to add 'Controller' and 'Form' in \Drupal\Core\DrupalKernel::compileContainer, as below:

if (!$component->isDot() && $component->isDir() && (
  is_dir($pathname . '/Command') ||
  is_dir($pathname . '/Controller') ||
  is_dir($pathname . '/Plugin') ||
  is_dir($pathname . '/Entity') ||
  is_dir($pathname . '/Element') ||
  is_dir($pathname . '/Form')
)) {
  $namespaces[$parent_namespace . '\\' . $component->getFilename()] = $path . '/' . $component->getFilename();
}

That would allow the following, and possibly others:

• BatchController
• CronController
• CsrfTokenController
• Http4xxController

We probably want to deprecate the old route names too, system.entity_autocomplete won't make sense once it's out of system module. Maybe core.entity_autocomplete would be a good replacement. Apparently we can use \Symfony\Component\Routing\Attribute\DeprecatedAlias in Route attributes to do this.

Postponed on #3585079: Convert remaining system module routes to use attributes