The InputDemo class uses the wrong service

Accessing /examples/form-api-example/input-demo causes the following exception.

Symfony\Component\DependencyInjection\Exception\ServiceNotFoundException: You have requested a non-existent service "module.extension.list". in Drupal\Component\DependencyInjection\Container->get() (line 159 of core/lib/Drupal/Component/DependencyInjection/Container.php).

Drupal\form_api_example\Form\InputDemo::create(Object) (Line: 41)
Drupal\Core\DependencyInjection\ClassResolver->getInstanceFromDefinition('\Drupal\form_api_example\Form\InputDemo') (Line: 48)
Drupal\Core\Controller\HtmlFormController->getFormObject(Object, '\Drupal\form_api_example\Form\InputDemo') (Line: 58)
Drupal\Core\Controller\FormController->getContentResult(Object, Object)
call_user_func_array(Array, Array) (Line: 123)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 638)
Drupal\Core\Render\Renderer->executeInRenderContext(Object, Object) (Line: 124)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->wrapControllerExecutionInRenderContext(Array, Array) (Line: 97)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 181)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1) (Line: 76)
Symfony\Component\HttpKernel\HttpKernel->handle(Object, 1, 1) (Line: 53)
Drupal\Core\StackMiddleware\Session->handle(Object, 1, 1) (Line: 48)
Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object, 1, 1) (Line: 28)
Drupal\Core\StackMiddleware\ContentLength->handle(Object, 1, 1) (Line: 32)
Drupal\big_pipe\StackMiddleware\ContentLength->handle(Object, 1, 1) (Line: 106)
Drupal\page_cache\StackMiddleware\PageCache->pass(Object, 1, 1) (Line: 85)
Drupal\page_cache\StackMiddleware\PageCache->handle(Object, 1, 1) (Line: 48)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1) (Line: 51)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1) (Line: 36)
Drupal\Core\StackMiddleware\AjaxPageState->handle(Object, 1, 1) (Line: 51)
Drupal\Core\StackMiddleware\StackedHttpKernel->handle(Object, 1, 1) (Line: 741)
Drupal\Core\DrupalKernel->handle(Object) (Line: 19)

The correct service is extension.list.module, but the class implementing that service is internal.

The only service which can be used is extension.path.resolver, which is not yet implemented in Drupal 11.

Either #2940481: Road to stabilize ExtensionList services or #2940190: [meta] Deprecations of old functions in Extension system could introduce changes that could influence which service we use in the Drupal\form_api_example\Form\InputDemo class. (So far, it seems those issues will not introduce changes in Drupal 10.x.)