Resolve #3408488 "Make unknownpathexcluder accept"

Closes #3408488

package_manager/src/PathExcluder/UnknownPathExcluder.php

@@ -66,14 +66,12 @@ final class UnknownPathExcluder implements EventSubscriberInterface {
    *   See \Drupal\package_manager\ComposerInspector::validate().
    */
   public function excludeUnknownPaths(CollectPathsToExcludeEvent $event): void {
-    $known_paths = [];
-
     $web_root = $this->pathLocator->getWebRoot();
     // If the web root and project root are the same, there's nothing to do.
     if (empty($web_root)) {
       return;
     }
-    $known_paths[] = $web_root;
+    $known_paths[] = [$web_root];
 
     $project_root = $this->pathLocator->getProjectRoot();
     // To determine the scaffold files to exclude, the installed packages must