The second find_package will cause error

[wsw364321644]

rapidjson/RapidJSONConfig.cmake.in

Lines 10 to 12 in 24b5e7a

	# RapidJSON build dir
	set( RapidJSON_DIR "@CONFIG_DIR@")

cmake 用<PackageName>_DIR 存储configuration file的目录。 RapidJSON_DIR被覆盖后会报错。