Ensure consistent spdlog dependency target no matter the source (#1101)

This removes the difference in targets generated for rmm depending on if spdlog is found on the machine, compared to being built as part of rmm.

Now in all cases rmm targets depend on `spdlog::spdlog_header_only` and never `rmm::spdlog_header_only`.

Authors:
  - Robert Maynard (https://github.com/robertmaynard)

Approvers:
  - Vyas Ramasubramani (https://github.com/vyasr)
  - Mark Harris (https://github.com/harrism)

URL: #1101

cmake/thirdparty/get_spdlog.cmake

@@ -17,11 +17,16 @@ function(find_and_configure_spdlog)
 
   include(${rapids-cmake-dir}/cpm/spdlog.cmake)
   rapids_cpm_spdlog(INSTALL_EXPORT_SET rmm-exports)
+  rapids_export_package(BUILD spdlog rmm-exports)
 
   if(spdlog_ADDED)
-    install(TARGETS spdlog_header_only EXPORT rmm-exports)
-  else()
-    rapids_export_package(BUILD spdlog rmm-exports)
+    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()
 endfunction()