Refactor gmock-cmake to make each phase step more consise.

There's now three distinct actions:

 - Importing some system library: _gmock_find_and_import_from_system
 - Importing from an external project: _add_external_project_with_gmock_cflags
 - Finding a source dir: _gmock_find_src_dirs_from_include_paths
Use standard FindPackageHandleStandardArgs and FindPackageMessage