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

[nalchevanidze]

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.

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 commercialhaskell/stackage#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'.

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

[nalchevanidze]

may stack clean --unlisted sound better