Moving check_package_boundaries to the presubmit checks directory

tools_webrtc/presubmit_checks_lib/testdata/multiple_errors_multiple_targets/BUILD.gn

 # Copyright (c) 2017 The WebRTC project authors. All Rights Reserved.
 #
 # Use of this source code is governed by a BSD-style license
 # that can be found in the LICENSE file in the root of the source
 # tree. An additional intellectual property rights grant can be found
 # in the file PATENTS.  All contributing project authors may
 # be found in the AUTHORS file in the root of the source tree.
 
 target("dummy_target_ok") {
   sources = [
     "dummy_source.cc",
     "dummy_source.h",
   ]
 }
 
 target("error_1") {
   sources = [
     "subpackage1/dummy_subpackage1.cc",
     "subpackage1/dummy_subpackage1.h",
   ]
 }
 
 target("error_2") {
   sources = [
     "subpackage1/dummy_subpackage2.cc",
     "subpackage1/dummy_subpackage2.h",
   ]
 }