widgets [nfc]: Strength-reduce flutter/cupertino imports to just flutter/widgets

Much like the previous commit.

Author: gnprice

test/model/autocomplete_test.dart

@@ -3,7 +3,7 @@ import 'dart:convert';
 
 import 'package:checks/checks.dart';
 import 'package:fake_async/fake_async.dart';
-import 'package:flutter/cupertino.dart';
+import 'package:flutter/widgets.dart';
 import 'package:test/scaffolding.dart';
 import 'package:zulip/api/model/initial_snapshot.dart';
 import 'package:zulip/api/model/model.dart';