Repacking a package that has been repacked throws:
```
Unhandled Exception: System.ArgumentException: Argument is not a JToken.
at Newtonsoft.Json.Linq.JContainer.EnsureValue(Object value)
at Newtonsoft.Json.Linq.JContainer.System.Collections.IList.Add(Object value)
at Inedo.ProGet.UPack.Repack.d__48.MoveNext()
```