More refactoring of package handling

- 'GhcPkg' now parses 'ghc-pkg -v list' output directly to also get the
  package-id

- Remove unused field 'cradlePackages' in Cradle

- Remove 'getPackageDbPackages' and use 'ghcPkgListEx' instead
Bring spec up to speed
Dont use Applicative ReadP instance
Merge pull request #223 from DanielG/dev

More refactoring of package handling