Fix generation of bundled.json, make source maps work in prod builds

Summary:
This diff fixes several issues around loading plugin, such as:

* make suresource maps work in the flipper-server production build
* make sure default plugins are no symlinked, which wouldn't work anywhere else but on the the system where it was build
* support release channel param for flipper-server

Bundled flipper-server is now 42MB (with icons (see later diffs) and plugins
```
ll flipper-server-v0.0.0.tgz
-rw-r--r--  1 mweststrate  staff    42M 23 Dec 15:29 flipper-server-v0.0.0.tgz
```

Reviewed By: nikoant

Differential Revision: D33294677

fbshipit-source-id: 63538dc8127f883fee6a3608673ad11ce239b350

desktop/flipper-server/src/startWebServerDev.tsx

@@ -17,7 +17,7 @@ import pFilter from 'p-filter';
 import {homedir} from 'os';
 
 // This file is heavily inspired by scripts/start-dev-server.ts!
-// part of that is done by start-flipper-server (compiling "main"),
+// part of that is done by start-flipper-server-dev (compiling "main"),
 // the other part ("renderer") here.
 
 const uiSourceDirs = [