Pass in platform argument in offline building

Summary: how did this ever work?
All build jobs must pass in the platform argument.
This also turns the "platform" argument into a required one.

I added a task to infer the platform argument from the filename here: t8306875

Reviewed By: @martinbigio

Differential Revision: D2425114

react-packager/src/Server/index.js

@@ -92,7 +92,7 @@ const bundleOpts = declareOpts({
   },
   platform: {
     type: 'string',
-    required: false,
+    required: true,
   }
 });