tty: refactor exports

This commit moves the tty module's exports to a single object,
which is more aligned with other core modules.

PR-URL: #16959
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

lib/tty.js

@@ -29,9 +29,9 @@ const errnoException = util._errnoException;
 const errors = require('internal/errors');
 const readline = require('readline');
 
-exports.isatty = function(fd) {
+function isatty(fd) {
   return Number.isInteger(fd) && fd >= 0 && isTTY(fd);
-};
+}
 
 
 function ReadStream(fd, options) {
@@ -54,8 +54,6 @@ function ReadStream(fd, options) {
 }
 inherits(ReadStream, net.Socket);
 
-exports.ReadStream = ReadStream;
-
 ReadStream.prototype.setRawMode = function(flag) {
   flag = !!flag;
   this._handle.setRawMode(flag);
@@ -90,7 +88,6 @@ function WriteStream(fd) {
   }
 }
 inherits(WriteStream, net.Socket);
-exports.WriteStream = WriteStream;
 
 
 WriteStream.prototype.isTTY = true;
@@ -131,3 +128,6 @@ WriteStream.prototype.clearScreenDown = function() {
 WriteStream.prototype.getWindowSize = function() {
   return [this.columns, this.rows];
 };
+
+
+module.exports = { isatty, ReadStream, WriteStream };