Feature/ `Stack clean --unused` cleans up all files that are not published

Sometimes we use Json or other related data files in our libraries.
They must be listed in `package.yml' `data-files', but we forgot about it.

```yml
data-files:
- test/**/*.gql
- test/**/*.json
```

since they exist in our git repository, it's not really possible to safely verify that all files are listed in `package.yml'.

This causes a problem like this https://github.com/commercialhaskell/stackage/issues/5602, the library is already released but is not built correctly.

We could easily avoid this scenario if we had such a code in our 'ci'.

```bash
stack clean --unused # cleans up all files that are not published
stack install --test
```