clean up dynamic plugin loading

Summary:
There are 3 sources where plugins can be loaded from:
* `src/plugins`
* `src/fb/plugins`
* any path specified in `~/.sonar/config.json`

Plugins found in the first two directories are bundled with the app when building.

Reviewed By: jknoxville

Differential Revision: D8636061

fbshipit-source-id: 2064090d43d11695ffd99df195e5b594559fe087

src/chrome/WelcomeScreen.js

@@ -15,7 +15,7 @@ import {
   colors,
   brandColors,
 } from 'sonar';
-import {isProduction} from '../utils/dynamicPluginLoading';
+import isProduction from '../utils/isProduction.js';
 import {shell, remote} from 'electron';
 
 const Container = FlexColumn.extends({