support user defined device plugins

Summary:
* move CPU and Logs plugin to plugins directory, set up package.json for them
* adjust plugins/index.js to expose device and client plugins in the same place, adding two new exports

Reviewed By: danielbuechele

Differential Revision: D10247606

fbshipit-source-id: 347bf8b3f9629987ad29d1d2ed025e0c88b9c967

src/devices/BaseDevice.js

@@ -6,7 +6,6 @@
  */
 
 import type stream from 'stream';
-import {FlipperDevicePlugin} from 'flipper';
 
 export type LogLevel =
   | 'unknown'