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/scripts/paths.ts

@@ -16,6 +16,10 @@ export const staticDir = path.join(rootDir, 'static');
 export const serverDir = path.join(rootDir, 'flipper-server');
 export const serverStaticDir = path.join(serverDir, 'static'); // for pre-bundled server, static resources are copied here
 export const defaultPluginsDir = path.join(staticDir, 'defaultPlugins');
+export const serverDefaultPluginsDir = path.join(
+  serverStaticDir,
+  'defaultPlugins',
+);
 export const pluginsDir = path.join(rootDir, 'plugins');
 export const fbPluginsDir = path.join(pluginsDir, 'fb');
 export const publicPluginsDir = path.join(pluginsDir, 'public');