[FEA] rapids_cpm_spdlog generate correct build config support

[robertmaynard]

Currently projects need to manually generate a build directory config module ( e.g. )

include(${rapids-cmake-dir}/cpm/spdlog.cmake)
  rapids_cpm_spdlog(FMT_OPTION "EXTERNAL_FMT_HO" INSTALL_EXPORT_SET rmm-exports)
  rapids_export_package(BUILD spdlog rmm-exports)

  if(spdlog_ADDED)
    rapids_export(
      BUILD spdlog
      EXPORT_SET spdlog
      GLOBAL_TARGETS spdlog spdlog_header_only
      NAMESPACE spdlog::)
    include("${rapids-cmake-dir}/export/find_package_root.cmake")
    rapids_export_find_package_root(BUILD spdlog [=[${CMAKE_CURRENT_LIST_DIR}]=] rmm-exports)
  endif()

Describe the solution you'd like

We should do this for projects in rapids_cpm_spdlog

[jameslamb]

Adding some links.

I think that instead of doing this for projects in rapids_cpm_spdlog(), we can soon just remove that manual exporting from projects' get_spdlog.cmake.

Assuming rapidsai/build-planning#56 is completed (see #592), across RAPIDS we'll be using spdlog>=1.13.0 , which contains @robertmaynard 's fix to its build exports to prevent the need for exactly this type of export management by projects that depend on spdlog: gabime/spdlog#2846.