hpack handler for cabal-install

It would be nice if cabal-install could detect a hpack package description file being present and called hpack in such cases, when it doesn't see a .cabal in the directory.

No file or test is named in the issue. Start by locating cabal-install's package-description detection entry point and inspect how it handles a directory without a .cabal file. Done means detecting the hpack package description and invoking hpack in that case, with coverage for the behavior.

Written by the indexing model from the issue text.