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/static/transforms/fb-stubs.js

@@ -10,7 +10,7 @@
 const fs = require('fs');
 const path = require('path');
 const replaceFBStubs = fs.existsSync(
-  path.join(__dirname, '..', '..', 'src', 'fb'),
+  path.join(__dirname, '..', '..', 'app', 'src', 'fb'),
 );
 const isFBFile = filePath => filePath.includes(`${path.sep}fb${path.sep}`);