Comprehensive CMake Package Config support

[MathiasMagnus]

This PR is part of a set of PRs implementing CMake Package Config detection support for all OpenCL repos independently and as part of the OpenCL SDK as well.

Related PRs:

• OpenCL-Headers
• OpenCL-CLHPP
• OpenCL-ICD-Loader

I tried keeping attribution where applicable with parallel PRs were in place.

Features in this PR:

• "External" (related Khronos projects) dependencies are pulled in as part of the build as well, in case someone did not clone recursively. The build clones to the same folder Git does, so they should not conflict, Git being smart enough.
• The build defines the IMPORTED targets I would advocate on keeping as CMake target names.
• I tried grabbing Ben's first sample and samples which are part of the build may look like:

  
  set(SAMPLE_NAME enumopencl)

add_executable(${SAMPLE_NAME} ${SAMPLE_NAME}.cpp)

target_link_libraries(${SAMPLE_NAME} PRIVATE OpenCL::ICD )

target_compile_definitions(${SAMPLE_NAME} PRIVATE CL_TARGET_OPENCL_VERSION=120 )

[KhronosWebservices]

Apologies again @MathiasMagnus would you please re-open this against 'main'.