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/Unit/Repository/VerifyTest.php

@@ -166,6 +166,10 @@ final class FlakyRangeBackend implements IBackend {
 		return $this->inner->list($prefix);
 	}
 
+	public function listFiles(string $dir): array {
+		return $this->inner->listFiles($dir);
+	}
+
 	public function delete(string $path): void {
 		$this->inner->delete($path);
 	}