Reland "Add a buildtools directory and move third_party/ninja to the project root in order to match the expectations of depot_tools" (#164240)

Relands https://github.com/flutter/flutter/pull/163890 and updates the
Ninja path used by the engine tool

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