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

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

third_party/ninja/README.flutter

@@ -0,0 +1,17 @@
+Name: ninja
+Short Name: ninja
+URL: https://github.com/ninja-build/ninja
+Revision: See the CIPD version in DEPS
+License: Apache License 2.0
+License File: https://github.com/ninja-build/ninja/blob/master/COPYING
+Security Critical: no
+
+Description:
+Ninja is a small build system with a focus on speed, and is used to build
+this project.
+
+The CIPD packages are built using the 3pp pipeline.
+https://chromium.googlesource.com/infra/infra/+/refs/heads/main/3pp/ninja/
+
+Local Modifications:
+None