Load external library versions using script instead of Git submodules.
SPIRV-Cross, Vulkan-LoaderAndValidationLayers & VulkanSamples versions recorded in files.
glslang version determined by Vulkan-LoaderAndValidationLayers.
SPIRV-Tools & SPIRV-Headers versions loaded by glslang.
When MoltenVK is built as part of the LunarG SDK, use external libraries
sourced from parent Vulkan-LoaderAndValidationLayers.
Use Vulkan headers from Vulkan-LoaderAndValidationLayers.
No longer generate Vulkan specification.
Update documentation.
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