find_package: Add path source to debug output

If cmake is called with the parameters --debug-find and --trace-expand, it is despite the verbose parameters not obvious from which source a path used by find_package comes.

Therefore, it would be very helpful to add a specification like <PackageName>_ROOT, <PackageName>_DIR, SYSTEM_PATH, REGISTRY, or similar to the log output, which describes the source according to the nine steps from CMake 3.18.2 Documentation - Search-Procedure.