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/Debug.hs

@@ -51,7 +51,9 @@ debug opt cradle fileName = do
     cabalFile = fromMaybe "" mCabalFile
     origGopts = ghcOpts opt
     simpleCompilerOption = CompilerOptions origGopts [] []
-    fromCabalFile = parseCabalFile file >>= getCompilerOptions origGopts cradle
+    fromCabalFile = do
+        pkgDesc <- parseCabalFile file
+        getCompilerOptions origGopts cradle pkgDesc
       where
         file = fromJust mCabalFile