CMake imported targets: create only one config file and add a namespace

[SpaceIm]

Currently, SPIRV-Cross doesn't seem to follow CMake best practices for imported targets:

• create one config file for the all library
• export a namespace for imported targets

It means that, when consuming SPIRV-Cross, we have to repeatedly call find_package for each component of SPIRV-Cross, as if they were independant projects, instead of just doing something like:

find_package(SPIRV-Cross REQUIRED spirv-cross-cpp spirv-cross-hlsl CONFIG) # only one config file required (for example SPIRV-Cross-Config.cmake)
add_executable(myapp myapp.cpp)
target_link_libraries(myapp PRIVATE SPIRV-Cross::spirv-cross-cpp SPIRV-Cross::spirv-cross-hlsl) # namespaced imported targets

[HansKristian-Work]

I'm not sure if I want to break existing clients' CMake integration which expect to see CMake targets being exposed in a specific way. As far as I can tell, this is a minor annoyance if anything?