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

DEPS

@@ -637,7 +637,7 @@ deps = {
     ],
     'dep_type': 'cipd',
   },
-  'engine/src/flutter/third_party/ninja': {
+  'third_party/ninja': {
     'packages': [
       {
         'package': 'infra/3pp/tools/ninja/${{platform}}',

buildtools/README.md

@@ -0,0 +1 @@
+This directory is a placeholder intended to make Flutter's source tree match the expectations of [depot_tools](https://www.chromium.org/developers/how-tos/depottools/), which is used to build the Flutter engine.

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',
   };

engine/src/flutter/ci/licenses_golden/excluded_files

@@ -2295,7 +2295,6 @@
 ../../../flutter/third_party/libwebp/swig
 ../../../flutter/third_party/libwebp/tests
 ../../../flutter/third_party/libwebp/webp_js
-../../../flutter/third_party/ninja
 ../../../flutter/third_party/ocmock
 ../../../flutter/third_party/perfetto/.clang-format
 ../../../flutter/third_party/perfetto/.clang-tidy

third_party/.gitignore

@@ -0,0 +1,9 @@
+# Ignore everything by default, as these come from gclient/DEPS.
+# We'll explicitly include the folders we want to track.
+/*
+
+# Include the .gitignore file itself.
+!.gitignore
+
+# Allow custom README.flutter files in each folder.
+**/README.flutter