Repository: list flat folders without a type check per entry

Opening a repository lists index/, which holds one file per pack.
StorageBackend::list() checked every entry with is_dir(), and on a
remote storage each check is a round trip. Over SFTP to a Hetzner Storage
Box (33 ms) with a 60 GiB repository, occ backup:run (30 s steps) spent
nearly every step on these requests and uploaded 45 KiB/s; with this
change it uploads 5.8 MB/s. The cost grows with the number of packs.

index/, snapshots/, catalog/ and trash/info/ only ever hold objects:
list them with the new IBackend::listFiles(), one listing without
per-entry checks.

tests/bootstrap.php

@@ -3,3 +3,13 @@
 declare(strict_types=1);
 
 require __DIR__ . '/../vendor/autoload.php';
+
+// nextcloud/ocp ships the public interfaces without an autoloader; tests that mock them need one.
+spl_autoload_register(static function (string $class): void {
+	if (str_starts_with($class, 'OCP\\')) {
+		$file = __DIR__ . '/../vendor/nextcloud/ocp/' . str_replace('\\', '/', $class) . '.php';
+		if (is_file($file)) {
+			require $file;
+		}
+	}
+});