Remove knowledge of fbjs from the packager

Summary:Follow-up to https://github.com/facebook/react-native/pull/5084

This…
- changes all requires within RN to `require('fbjs/lib/…')`
- updates `.flowconfig`
- updates `packager/blacklist.js`
- adapts tests
- removes things from `Libraries/vendor/{core,emitter}` that are also in fbjs
- removes knowledge of `fbjs` from the packager

Closes https://github.com/facebook/react-native/pull/5084

Reviewed By: bestander

Differential Revision: D2926835

fb-gh-sync-id: 2095e22b2f38e032599d1f2601722b3560e8b6e9
shipit-source-id: 2095e22b2f38e032599d1f2601722b3560e8b6e9

Libraries/JavaScriptAppEngine/System/JSTimers/JSTimersExecution.js

@@ -10,10 +10,10 @@
  */
 'use strict';
 
-var invariant = require('invariant');
-var keyMirror = require('keyMirror');
-var performanceNow = require('performanceNow');
-var warning = require('warning');
+var invariant = require('fbjs/lib/invariant');
+var keyMirror = require('fbjs/lib/keyMirror');
+var performanceNow = require('fbjs/lib/performanceNow');
+var warning = require('fbjs/lib/warning');
 var Systrace = require('Systrace');
 
 /**