Move layout plugin sidebar extensions to the plugin package

Summary: Moved layout plugin sidebar extensions to the plugin package.

Reviewed By: passy

Differential Revision: D27036489

fbshipit-source-id: 5b3406e5c50ba61709d925a3c0cb07a6fd7552db

desktop/plugins/public/layout/InspectorSidebar.tsx

@@ -14,7 +14,6 @@ import {
   styled,
   colors,
   PluginClient,
-  SidebarExtensions,
   Element,
   Client,
   Logger,
@@ -24,6 +23,7 @@ import deepEqual from 'deep-equal';
 import React from 'react';
 import {useMemo, useEffect} from 'react';
 import {kebabCase} from 'lodash';
+import {default as SidebarExtensions} from './extensions/fb-stubs/index';
 
 const NoData = styled(FlexCenter)({
   fontSize: 18,