code: def R_package_resource(package, resource):
      
      package_path = R_package_path(package)
      if not package_path:
          return None
      package_resource = os.path.join(package_path, resource)
      if not file_exists(package_resource):
          return None
      else:
          return package_resource
docs: return a path to an R package resource, if it is available