This makes the BUILD.gn source lists a lot simpler, since they no
longer need to use a special path variable, but instead can use the
correct relative paths.

This will lead to further simplifications so we can hopefully upstream
the Vulkan tools BUILD.gn files.

Second part updates the build files and removes the old repositories.

Bug: angleproject:2339
Change-Id: I37f5b42a0854ca49448ecbbb32c19c24df57f452
Reviewed-on: https://chromium-review.googlesource.com/905894Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>