Add a buildtools directory and move third_party/ninja to the project root in order to match the expectations of depot_tools (#163890)

The depot_tools Ninja wrapper looks for Ninja at specific paths within
the tree. The use of "name": "." in the engine's .gclient file is not
expected by the depot_tools logic for locating the engine's source
directory.

This PR creates an alternative layout that is supported by depot_tools.
If the project root directory contains a "buildtools" subdirectory, then
depot_tools will look for Ninja at "third_party/ninja" within the root.

Fixes https://github.com/flutter/flutter/issues/163487

dev/bots/analyze.dart

@@ -1632,6 +1632,7 @@ Future<void> verifyRepositoryLinks(String workingDirectory) async {
     'glfw/glfw',
     'GoogleCloudPlatform/artifact-registry-maven-tools',
     'material-components/material-components-android', // TODO(guidezpl): remove when https://github.com/material-components/material-components-android/issues/4144 is closed
+    'ninja-build/ninja',
     'torvalds/linux',
     'tpn/winsdk-10',
   };