Enabling JavaConversions together with FlatPackage breaks compilation

See title:

This works:
```
scalapbCodeGeneratorOptions ++= Seq(FlatPackage)
```

This doesn't:
```
scalapbCodeGeneratorOptions ++= Seq(FlatPackage , JavaConversions)
```

Note: I'm using features such as `oneof`, so it might be a special case. Let me know if you need more information to reproduce this. I'll also look into it when I have a bit more spare time. 