The `Vulkan::Headers` target complements existing Vulkan::Vulkan target.
It is the same except it omits the Vulkan library which supports
applications that loads the Vulkan library in at runtime.
The `Vulkan::glslangValidator` target provides the glslangValidator
executable which is the tool for converting between shader languages
(GLSL, SPIR-V, etc.).
The GLSL SPIR-V compiler is part of the Vulkan SDK and may be used
by projects for compiling shaders as part of the build process.
This is not strictly required to build a Vulkan application, which
is why the variable is not part of the REQUIRED_VARs for the module.
