Upstream removed the option in KhronosGroup/Vulkan-Loader#260, which
breaks our current use case.
This commit reverts KhronosGroup/Vulkan-Loader#260 is our vendored
loader.
We may need to re-evaluate how we link the loader, but until then,
reverting this PR fixes Windows support after the upgrade to a recent
SDK version in #36932.
- 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.