function _moduleRootAndHandler(fullHandlerString) {
  const handlerString = path.basename(fullHandlerString)
  const moduleRoot = fullHandlerString.substring(
    0,
    fullHandlerString.indexOf(handlerString),
  )
  return [moduleRoot, handlerString]
}
Break the full handler string into two pieces, the module root and the actual
handler string.
Given './somepath/something/module.nestedobj.handler' this returns
['./somepath/something', 'module.nestedobj.handler']