Add support for new api "vulkanbase" (KhronosGroup#2323)

* Add support for api "vulkanbase"

* Merge internal APIs before processing

After loading the XML, merge internal API blocks into the public ones
so that we don't litter the vulkan and vulkan sc headers with
Base/Compute/Graphics version notations.

* Modify algorithm to merge internal features into public ones.

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Co-authored-by: Daniel Koch <dkoch@nvidia.com>

Author: asuessenbach