Move pub into sdk/lib/_internal.

sdk/lib/_internal/pub/test/install/path/absolute_path_test.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS d.file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE d.file.
 
 import 'package:pathos/path.dart' as path;
 import 'package:scheduled_test/scheduled_test.dart';
 
-import '../../../../pub/io.dart';
+import 'package:pub/src/io.dart';
 import '../../descriptor.dart' as d;
 import '../../test_pub.dart';
 
 main() {
   initConfig();
   integration('path dependency with absolute path', () {
     d.dir('foo', [
       d.libDir('foo'),
       d.libPubspec('foo', '0.0.1')
     ]).create();
@@ skipping 21 matching lines @@
     d.dir("moved").create();
     scheduleRename(packagesPath, "moved/packages");
 
     d.dir("moved/packages", [
       d.dir("foo", [
         d.file("foo.dart", 'main() => "foo";')
       ])
     ]).validate();
   });
 }