@import doesn't seem to work with --chugin flag

We tried doing this with a local build of `Chumpinate`:

```
> chuck build-pkg-mac.ck --chugin:../Chumpinate.chug
```

where `build-pkg-mac.ck` imports `Chumpinate`:

```
@import "Chumpinate"
```

and it's unable to find this chugin file.