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>

third_party/.gitignore

@@ -1,9 +0,0 @@
-# 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

@@ -1,17 +0,0 @@
-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