build: move build scripts to dedicated directory (#35780)

This commit moves the build-related scripts
(`build-ivy-npm-packages.js`, `build-packages-dist.js` and
`package-builder.js`) to a dedicated directory to keep the `scripts/`
directory cleaner.

It also moves the logic for building the `zone.js` package to a separate
script, `zone-js-builder.js`, to make it re-usable. A subsequent commit
will use it to build the `zone.js` package when building the Ivy Angular
packages as well.

PR Close #35780

scripts/build-package-dist.js

@@ -0,0 +1,27 @@
+#!/usr/bin/env node
+/**
+ * @license
+ * Copyright Google Inc. All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ */
+
+// The build script used to be here but has been moved to `scripts/build/` in PR
+// https://github.com/angular/angular/pull/35780. This is a temporary placeholder script for people
+// that are not aware of the change and expect to find the script here.
+//
+// TODO: This script can be removed early May 2020.
+'use strict';
+
+const {red} = require('chalk');
+const {relative, resolve} = require('path');
+
+
+const absoluteScriptPath = resolve(`${__dirname}/build/build-packages-dist.js`);
+const relativeScriptPath = relative(process.cwd(), absoluteScriptPath);
+
+console.error(red('ERROR: The build script has been moved to \'scripts/build/\'.'));
+console.error(red(`       Run: node ${relativeScriptPath}`));
+
+process.exit(1);