build: Refactor gulpfiles from CJS to ESM (#9149)

* refactor(build): Rename "package" gulp task (but not npm script) to "pack"

  This is to avoid an issue due to "package" being a reserved word
  in JavaScript, and therefore not a valid export identifier.

* refactor(build): Convert gulpfile.js from CJS to ESM.

* refactor(build): Convert scripts/gulpfiles/*.js from CJS to ESM

* fix(build): Fix eslint warning for @license tag in gulpfile.mjs

* chore(build): Remove unused imports

* fix(build): Fix incorrect import of gulp-gzip

* fix(build): Fix incorrect sourcemaps import reference

gulpfile.mjs

@@ -0,0 +1,95 @@
+/**
+ * @license
+ * Copyright 2018 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+/**
+ * @fileoverview Gulp script to build Blockly for Node & NPM.
+ * Run this script by calling "npm install" in this directory.
+ */
+/* eslint-env node */
+
+// Needed to prevent prettier from munging exports order, due to
+// https://github.com/simonhaenisch/prettier-plugin-organize-imports/issues/146
+// - but has the unfortunate side effect of suppressing ordering of
+// imports too:
+//
+// organize-imports-ignore
+
+import {parallel} from 'gulp';
+import {
+  deployDemos,
+  deployDemosBeta,
+  prepareDemos,
+} from './scripts/gulpfiles/appengine_tasks.mjs';
+import {
+  build,
+  buildAdvancedCompilationTest,
+  cleanBuildDir,
+  langfiles,
+  messages,
+  minify,
+  tsc,
+} from './scripts/gulpfiles/build_tasks.mjs';
+import {docs} from './scripts/gulpfiles/docs_tasks.mjs';
+import {
+  createRC,
+  syncDevelop,
+  syncMaster,
+  updateGithubPages,
+} from './scripts/gulpfiles/git_tasks.mjs';
+import {cleanReleaseDir, pack} from './scripts/gulpfiles/package_tasks.mjs';
+import {
+  publish,
+  publishBeta,
+  recompile,
+} from './scripts/gulpfiles/release_tasks.mjs';
+import {generators, test} from './scripts/gulpfiles/test_tasks.mjs';
+
+const clean = parallel(cleanBuildDir, cleanReleaseDir);
+
+// Default target if gulp invoked without specifying.
+export default build;
+
+// Main sequence targets.  They already invoke prerequisites.  Listed
+// in typical order of invocation, and strictly listing prerequisites
+// before dependants.
+//
+// prettier-ignore
+export {
+  langfiles,
+  tsc,
+  minify,
+  build,
+  pack,  // Formerly package.
+  publishBeta,
+  publish,
+  prepareDemos,
+  deployDemosBeta,
+  deployDemos,
+  updateGithubPages as gitUpdateGithubPages,
+}
+
+// Manually-invokable targets that also invoke prerequisites where
+// required.
+//
+// prettier-ignore
+export {
+  messages, // Generate msg/json/en.json et al.
+  clean,
+  test,
+  generators as testGenerators,
+  buildAdvancedCompilationTest,
+  createRC as gitCreateRC,
+  docs,
+}
+
+// Legacy targets, to be deleted.
+//
+// prettier-ignore
+export {
+  recompile,
+  syncDevelop as gitSyncDevelop,
+  syncMaster as gitSyncMaster,
+}