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

test/Main.hs

@@ -0,0 +1,12 @@
+import Spec
+import Dir
+
+import Test.Hspec
+import System.Process
+
+main = do
+  let sandboxes = [ "test/data", "test/data/check-packageid" ]
+      genSandboxCfg dir = withDirectory dir $ \cwd -> do
+         system ("sed 's|@CWD@|" ++ cwd ++ "|g' cabal.sandbox.config.in > cabal.sandbox.config")
+  genSandboxCfg `mapM` sandboxes
+  hspec spec