Yarn workspaces

Summary:
1) moved "sonar/desktop/src" to "sonar/desktop/app/src", so "app" is now a separate package containing the core Flipper app code
2) Configured yarn workspaces with the root in "sonar/desktop": app, static, pkg, doctor, headless-tests. Plugins are not included for now, I plan to do this later.

Reviewed By: jknoxville

Differential Revision: D20535782

fbshipit-source-id: 600b2301960f37c7d72166e0d04eba462bec9fc1

desktop/plugins/crash_reporter/index.js

@@ -35,7 +35,7 @@ import os from 'os';
 import util from 'util';
 import path from 'path';
 import {promisify} from 'util';
-import type {Notification} from '../../src/plugin.tsx';
+import type {Notification} from 'flipper';
 import type {Store, DeviceLogEntry, OS, Props} from 'flipper';
 import {Component} from 'react';