Issue #3445713 - Add new interface

Closes #3445713

src/UnusedModulesHelperService.php

@@ -9,13 +9,10 @@ use Drupal\Core\Extension\ExtensionDiscovery;
 /**
  * Common Unused Modules functionality.
  */
-class UnusedModulesHelperService {
+class UnusedModulesHelperService implements UnusedModulesHelperServiceInterface {
 
   /**
-   * Returns an array with all available modules.
-   *
-   * @return \Drupal\unused_modules\UnusedModulesExtensionDecorator[]
-   *   The list of available modules.
+   * {@inheritdoc}
    */
   public function getModulesByProject() {
     $enabled_modules = static::getEnabledModules();