- Renamed option to `builtin_vulkan`, since that's the name of the
  library and if we were to add new components, we'd likely use that
  same option.
- Merge `vulkan_loader/SCsub` in `vulkan/SCsub`.
- Accordingly, don't use built-in Vulkan headers when not building
  against the built-in loader library.
- Drop Vulkan registry which we don't appear to need currently.
- Style and permission fixes.