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
