migrate to "export default"

also, set no-use-before-define in linter

src/backupformats.js

@@ -1,12 +1,8 @@
 import assert from 'node:assert';
 import BoxError from './boxerror.js';
-import * as tgzFormat from './backupformat/tgz.js';
-import * as rsyncFormat from './backupformat/rsync.js';
+import tgzFormat from './backupformat/tgz.js';
+import rsyncFormat from './backupformat/rsync.js';
 
-export {
-    api,
-    validateFormat,
-};
 
 const FORMATS = { tgz: tgzFormat, rsync: rsyncFormat };
 
@@ -25,3 +21,8 @@ function validateFormat(format) {
 
     return new BoxError(BoxError.BAD_FIELD, 'Invalid backup format');
 }
+
+export default {
+    api,
+    validateFormat,
+};