storage: make listDir return paths relative to the root

this seems more natural to work with

src/storage/interface.js

@@ -101,7 +101,7 @@ async function listDir(apiConfig, dir, batchSize, marker) {
     assert(typeof marker !== 'undefined');
 
     // Result: array of { path, size }
-    // path is relative to the dir being listed
+    // path is relative to the prefix/root and not the _dir_ being listed
     throw new BoxError(BoxError.NOT_IMPLEMENTED, 'listDir is not implemented');
 }