This was only necessary when the Engine had to build in multiple buildroots
where the sources where checked out at different paths relative to the
buildroot. This is no longer the case and there are already cases GN rules
have been written that mix and match variable usage with the direct
specification of the path to the Flutter sources relative to the sole buildroot.

Final portion of reworking the names of the Linux and Windows desktop build outputs.

See flutter/flutter#38589

Part of restructuring the artifacts to support transitioning away from
GLFW embeddings on desktop.

https://github.com/flutter/flutter/issues/38589

Start work on flutter/flutter#30726 by adding an alternative win32 shell platform implementation for Windows that is not based on GLFW and that uses LIBANGLE for rendering and native win32 windowing and input. This change does not replace the GLFW implementation but rather runs side by side with it producing a secondary flutter_windows_win32.dll artifact. The following items must be added to attain parity with the GLFW implementation:
- Custom task scheduling
- Support for keyboard modifier keys
- Async texture uploads
- Correct high DPI handling on Windows versions < 1703
and will be added in subsequent changes.

Linux targets building the embedder library may be targeting environments
that do not support GLFW and GTK.

Enables building of the GLFW shell and the related unit tests.

Changes include:
- File structure
- Header guards
- Include paths
- Namespaces
- Integration with the engine's GN build
- Conversion from jsoncpp to rapidjson
- Style and clang-format adjustment to match engine repository

The Fuchsia bots seem to reference this target to figure out the root_out_directory https://fuchsia.googlesource.com/build/+/master/dart/dart_test.gni#67. Note that the presence of the flutter_tester binary location itself is not depended on. Instead, the target is used to infer the directory containing the flutter_tester after a build.

Patching the //build repository in Fuchsia would mean making the tree red till a //topaz patch lands. To avoid doing this, we add back the missing reference.

They help the analysis service identify packages and are now mandatory on Fuchsia.