Move desktop-related code to "desktop" subfolder (#872)

Summary:
Pull Request resolved: https://github.com/facebook/flipper/pull/872
Move all the JS code related to desktop app to "desktop" subfolder.

The structure of "desktop" folder:
- `src` - JS code of Flipper desktop app executing in Electron Renderer (Chrome) process. This folder also contains all the Flipper plugins in subfolder "src/plugins".
- `static` - JS code of Flipper desktop app bootstrapping executing in Electron Main (Node.js) process
- `pkg` - Flipper packaging lib and CLI tool
- `doctor` - Flipper diagnostics lib and CLI tool
- `scripts` - Build scripts for Flipper desktop app
- `headless` - Headless version of Flipper app
- `headless-tests` - Integration tests running agains Flipper headless version

Reviewed By: passy

Differential Revision: D20249304

fbshipit-source-id: 9a51c63b51b92b758a02fc8ebf7d3d116770efe9

desktop/pkg/src/utils/yarn.ts

@@ -0,0 +1,36 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @format
+ */
+
+import * as util from 'util';
+import {exec as execImport} from 'child_process';
+const exec = util.promisify(execImport);
+
+const WINDOWS = /^win/.test(process.platform);
+const YARN_PATH = 'yarn' + (WINDOWS ? '.cmd' : '');
+
+export async function install(pkgDir: string) {
+  const {stderr} = await exec(YARN_PATH, {
+    cwd: pkgDir,
+  });
+  if (stderr) {
+    console.warn(stderr);
+  }
+}
+
+export async function pack(pkgDir: string, out: string) {
+  const {stderr} = await exec(
+    [YARN_PATH, 'pack', '--filename', out].join(' '),
+    {
+      cwd: pkgDir,
+    },
+  );
+  if (stderr) {
+    console.warn(stderr);
+  }
+}