Transitive import (barrel files)

It would be great to be able to import `.zmodel` files that import other files.

```ts
import "./schema/base/index.zmodel"
```

Contents of `index.zmodel`:

```ts
import "./user.zmodel"
import "./push-subscription.zmodel"
import "./chat.zmodel"
import "./message.zmodel"
import "./media.zmodel"
```