Refactor the way packages databases are handled

- cradle now stores a list of active package databases instead of only
  the user store (if present).

- rename `cradlePackageDb` -> `cradlePkgDbStack` as that`s what the ghc
  documentaion calls this kind of thing

- `getPackageDbPackages` now returns names of all visible packages in
  the given directory. Also the implementation now uses `ghc-pkg`
  instead of manually looking at the package database

Language/Haskell/GhcMod/Internal.hs

@@ -7,9 +7,6 @@ module Language.Haskell.GhcMod.Internal (
   , Package
   , IncludeDir
   , CompilerOptions(..)
-  -- * Cradle
-  , userPackageDbOptsForGhc
-  , userPackageDbOptsForGhcPkg
   -- * Cabal API
   , parseCabalFile
   , getCompilerOptions
@@ -38,7 +35,6 @@ module Language.Haskell.GhcMod.Internal (
 
 import Language.Haskell.GhcMod.Browse
 import Language.Haskell.GhcMod.CabalApi
-import Language.Haskell.GhcMod.Cradle
 import Language.Haskell.GhcMod.ErrMsg
 import Language.Haskell.GhcMod.GHCApi
 import Language.Haskell.GhcMod.GHCChoice