Script pack usings don't appear in :usings

``` batch
mkdir bug-repro
cd bug-repro
scriptcs -install ScriptCs.Nancy
scriptcs
```

``` csharp
var n = Require<NancyPack>();
class TestModule: NancyModule { } // prove that the Nancy namespace is available
:usings
```

``` json
[
  "System",
  "System.Collections.Generic",
  "System.Linq",
  "System.Text",
  "System.Threading.Tasks",
  "System.IO",
  "System.Net.Http"
]
```
