error re-adding a package

Adding a package twice gives:

```
INFO: Nothing to be done.
ERROR: type: non-boolean (Nothing) used in boolean context
 in add at pkg/entry.jl:22
 in anonymous at pkg/dir.jl:31
 in cd at file.jl:22
 in cd at pkg/dir.jl:31
 in add at pkg.jl:14
```

cc: @StefanKarpinski 
