Switch to .mjs for ES module output for Node.js compat

This also reverts commit 60cf26c2f3.

package.json

@@ -2,11 +2,12 @@
   "name": "node-fetch",
   "version": "2.1.2",
   "description": "A light-weight module that brings window.fetch to node.js",
-  "main": "lib/index.js",
+  "main": "lib/index",
   "browser": "./browser.js",
-  "module": "lib/index.es.js",
+  "module": "lib/index.mjs",
   "files": [
     "lib/index.js",
+    "lib/index.mjs",
     "lib/index.es.js",
     "browser.js"
   ],
@@ -47,6 +48,7 @@
     "codecov": "^3.0.0",
     "cross-env": "^5.1.3",
     "form-data": "^2.3.1",
+    "is-builtin-module": "^1.0.0",
     "mocha": "^5.0.0",
     "nyc": "11.9.0",
     "parted": "^0.1.1",

rollup.config.js

@@ -1,3 +1,4 @@
+import isBuiltin from 'is-builtin-module';
 import babel from 'rollup-plugin-babel';
 import tweakDefault from './build/rollup-plugin';
 
@@ -7,7 +8,8 @@ export default {
   input: 'src/index.js',
   output: [
     { file: 'lib/index.js', format: 'cjs', exports: 'named' },
-    { file: 'lib/index.es.js', format: 'es', exports: 'named' }
+    { file: 'lib/index.es.js', format: 'es', exports: 'named', intro: 'process.emitWarning("The .es.js file is deprecated. Use .mjs instead.");' },
+    { file: 'lib/index.mjs', format: 'es', exports: 'named' },
   ],
   plugins: [
     babel({
@@ -16,6 +18,9 @@ export default {
     tweakDefault()
   ],
   external: function (id) {
+    if (isBuiltin(id)) {
+      return true;
+    }
     id = id.split('/').slice(0, id[0] === '@' ? 2 : 1).join('/');
     return !!require('./package.json').dependencies[id];
   }

src/body.js

@@ -5,12 +5,10 @@
  * Body interface provides common methods for Request and Response
  */
 
+import Stream, { PassThrough } from 'stream';
 import Blob, { BUFFER } from './blob.js';
 import FetchError from './fetch-error.js';
 
-const Stream = require('stream');
-const { PassThrough } = require('stream');
-
 let convert;
 try { convert = require('encoding').convert; } catch(e) {}
 

src/index.js

@@ -7,18 +7,18 @@
  * All spec algorithm step numbers are based on https://fetch.spec.whatwg.org/commit-snapshots/ae716822cb3a61843226cd090eefc6589446c1d2/.
  */
 
+import { resolve as resolve_url } from 'url';
+import http from 'http';
+import https from 'https';
+import zlib from 'zlib';
+import { PassThrough } from 'stream';
+
 import Body, { writeToStream, getTotalBytes } from './body';
 import Response from './response';
 import Headers, { createHeadersLenient } from './headers';
 import Request, { getNodeRequestOptions } from './request';
 import FetchError from './fetch-error';
 
-const http = require('http');
-const https = require('https');
-const { PassThrough } = require('stream');
-const { resolve: resolve_url } = require('url');
-const zlib = require('zlib');
-
 /**
  * Fetch function
  *

src/request.js

@@ -7,11 +7,10 @@
  * All spec algorithm step numbers are based on https://fetch.spec.whatwg.org/commit-snapshots/ae716822cb3a61843226cd090eefc6589446c1d2/.
  */
 
+import { format as format_url, parse as parse_url } from 'url';
 import Headers, { exportNodeCompatibleHeaders } from './headers.js';
 import Body, { clone, extractContentType, getTotalBytes } from './body';
 
-const { format: format_url, parse: parse_url } = require('url');
-
 const INTERNALS = Symbol('Request internals');
 
 /**

src/response.js

@@ -5,11 +5,10 @@
  * Response class provides content decoding
  */
 
+import { STATUS_CODES } from 'http';
 import Headers from './headers.js';
 import Body, { clone } from './body';
 
-const { STATUS_CODES } = require('http');
-
 const INTERNALS = Symbol('Response internals');
 
 /**