Automatic porting of ebuilds from `::haskell` to `::gentoo`

[hololeap]

The basic idea is to add a new mode to hackport, which (at the minimum) calculates a minimum set of ebuilds needed to port current major packages from ::haskell to ::gentoo, while meeting Gentoo's QA standards.

The required parameters would be:

1. One or more packages that are considered primary targets for porting to ::gentoo

   • These have been referred to as "leaf packages" in the past.
   • See https://github.com/gentoo-haskell/gentoo-haskell/wiki/Leaf-packages-in-main-gentoo-repo

2. One or more target GHC versions that will be supported in ::gentoo

   • Some packages are finicky about what GHC versions they support
     • For instance, each major release of Cabal supports a maximum GHC version
     • Each release of ghc-exactprint supports only a single major release of GHC
   • This can possibly be calculated using existing profiles/package.mask/ and scripts/package.unmask/, but it is likely necessary to add metadata about supported GHC versions to ebuilds that need it (as #hackport comments).