Documentation: Package with version on local file system

It might be good to extend the documentation to clarify the structure of the  `deps` directory in case of packages with a version number.

I believe the structure is
```
deps/
├─ <namespace>/
│  ├─ <package>/
│  │  ├─ <version>/
│  │  │  ├─ a.wit
│  │  │  ├─ ...
``` 

