This points to something weird: if I do `dumpdata --pks 2,3,4 app` I'll get _any_ object of any type with PKs of 2, 3, or 4. That's... weird. I think a better approach would be to make `--pks` only work if you've specified a _single_ model. So `dumpdata --pks 2,3,4 app.Model` will work, but `dumpdata --pks 2,3,4 app app2` would through an error.