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/build-release.ts

@@ -31,7 +31,7 @@ import {
 import fetch from '@adobe/node-fetch-retry';
 import isFB from './isFB';
 import copyPackageWithDependencies from './copy-package-with-dependencies';
-import {staticDir, distDir} from './paths';
+import {staticDir, distDir, defaultPluginsDir} from './paths';
 import yargs from 'yargs';
 import {WinPackager} from 'app-builder-lib/out/winPackager';
 // eslint-disable-next-line node/no-extraneous-import