private static String setterFieldNameOrNull(String methodName) {
        if (methodName.startsWith("set")) {
            return methodName.substring(3);
        }
        return null;
    }
Returns a Multimap from fieldName Strings to their getter Methods.

It expects to be called only from TaskSerDe. Multimap is used inside org.embulk.config.
embulk-deps/src/main/java/org/embulk/deps/config/TaskInvocationHandler.java