Update targets only included in host_debug_unopt.

This reverts commit f5d0004e.

This is intended to match the exit codes returned by the Dart command line tool

After this patch, we no longer link with Mojo. We still use some gn definitions
from //mojo to create Dart packages.

We no longer use mojom to transport messages. We still use the Mojo EDK
to spin the event loop, however.

This patch moves the rest of the clients to use the new PointerData
transport.

* Namespaces have been updated to reflect the move from //flutter/sky/shell to //flutter/shell.
* shell/BUILD.gn file has been split into smaller GN files for each subcomponent of the shell (common, GPU, diagnostic, testing).
* GN dependencies have been rewritten to stop exposing common shell dependencies as public. Duplicates have also been removed.
* GPU subcomponent has been updated make it more suitable for Vulkan integration.
* The GLFW backend has been resurrected.