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/flow-typed/flipper.js

@@ -43,6 +43,8 @@ declare module 'flipper' {
     supportsMethod(method: string): Promise<boolean>;
   }
 
+  declare type Logger = Logger;
+
   declare type PluginTarget = BaseDevice | Client;
 
   declare type Notification = {|