Check if the absolute path is a directory when scanning for static files

and not if the relative one is an directory
This caused the welcome page not to be found in sub directories,
since the KnownPathResourceManager did not know about this specific
folder, and did not return a DirectoryResource
fixes quarkusio#3091

extensions/undertow/deployment/src/main/java/io/quarkus/undertow/deployment/UndertowStaticResourcesBuildStep.java

@@ -60,7 +60,7 @@ public void accept(Path path) {
                             return;
                         }
                         Path rel = resource.relativize(path);
-                        if (Files.isDirectory(rel)) {
+                        if (Files.isDirectory(path)) {
                             knownDirectories.add(rel.toString());
                         } else {
                             knownFiles.add(rel.toString());