Remove package root target and support scripts

pkg/BUILD.gn

-# Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS 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 file.
-
-action("pkg") {
-  # TODO(zra): Add third_party/pkg, third_party/pkg_tested, and
-  # third_party/observatory_pub_packages/packages/charted/lib.
-  list_script = "../tools/list_pkg_directories.py"
-  pkg_list = exec_script(list_script, [rebase_path(".")], "list lines")
-  runtime_list = exec_script(
-      list_script, [rebase_path("../runtime")], "list lines")
-
-  inputs = pkg_list +
-           runtime_list +
-           [rebase_path("../sdk/lib/_internal/js_runtime/lib"),
-            rebase_path("../sdk/lib/_internal/sdk_library_metadata/lib"),]
-
-  timestamp_file = "$target_gen_dir/packages.stamp"
-  outputs = [
-    timestamp_file,
-    "$root_out_dir/.packages",
-  ]
-
-  script = "../tools/make_links.py"
-  args = [
-    "--create-links",
-    "--create-package-file",
-    "--quiet",
-    "--timestamp_file",
-    rebase_path(timestamp_file),
-  ] + inputs +
-    # Pub imports dart2js as compiler_unsupported so it can work outside
-    # the SDK. Map that to the compiler package.
-    [rebase_path("compiler/lib") + ":compiler_unsupported",]
-}