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-core/src/FlipperServerImpl.tsx

@@ -81,6 +81,9 @@ export class FlipperServerImpl implements FlipperServer {
     keytarModule?: KeytarModule,
   ) {
     setFlipperServerConfig(config);
+    console.log(
+      'Loaded flipper config, paths: ' + JSON.stringify(config.paths, null, 2),
+    );
     const server = (this.server = new ServerController(this));
     this.android = new AndroidDeviceManager(this);
     this.ios = new IOSDeviceManager(this);