Reverts "Add a buildtools directory and move third_party/ninja to the project root in order to match the expectations of depot_tools (#163890)" (#164209)

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Reverts: flutter/flutter#163890
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Initiated by: jason-simmons
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Reason for reverting: et and related packages in
https://github.com/flutter/flutter/tree/master/engine/src/flutter/tools
need to be updated for the new Ninja path
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Original PR Author: jason-simmons
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Reviewed By: {matanlurey, jtmcdole}
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This change reverts the following previous change:
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
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Co-authored-by: auto-submit[bot] <flutter-engprod-team@google.com>

dev/bots/analyze.dart

@@ -1632,7 +1632,6 @@ 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',
   };