function _splitHandlerString(handler) {
  const match = handler.match(FUNCTION_EXPR)
  if (!match || match.length !== 3) {
    throw new MalformedHandlerName("Bad handler")
  }
  return [match[1], match[2]] // [module, function-path]
}
Split the handler string into two pieces: the module name and the path to
the handler function.