Yeah, unfortunately 4.20 is missing the .uplugin file in GoogleARCoreServices plugin, which is why the folder only contains the source code folder. The workaround for now is to use the source code version of 4.20 and manually added a GoogleARCoreServices.uplugin (https://github.com/google-ar-unreal/UnrealEngine/blob/4.19.1-arcore-v1.2.1/Engine/Plugins/Runtime/GoogleARCoreServices/GoogleARCoreServices.uplugin) file in that folder. Once you added the .uplugin file, generate the project file and build the engine again, you should be able to use the GoogleARCoreServices plugin for CloudARPin.
If you want to use binary version of Unreal 4.20, you will have to copy the GoogleARCoreServices folder to a project plugin and add the uplugin there.