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/GhcPkgSpec.hs

@@ -0,0 +1,27 @@
+module GhcPkgSpec where
+
+import Language.Haskell.GhcMod.Types
+import Language.Haskell.GhcMod.GhcPkg
+
+import Control.Applicative
+import System.Directory
+import System.FilePath ((</>))
+import Test.Hspec
+
+import Dir
+
+spec :: Spec
+spec = do
+    describe "getSandboxDb" $ do
+        it "parses a config file and extracts sandbox package db" $ do
+            cwd <- getCurrentDirectory
+            pkgDb <- getSandboxDb "test/data/"
+            pkgDb `shouldBe` (cwd </> "test/data/.cabal-sandbox/i386-osx-ghc-7.6.3-packages.conf.d")
+
+        it "throws an error if a config file is broken" $ do
+            getSandboxDb "test/data/broken-sandbox" `shouldThrow` anyException
+
+    describe "getPackageDbPackages" $ do
+        it "find a config file and extracts packages" $ do
+            pkgs <- getPackageDbPackages "test/data/check-packageid"
+            pkgs `shouldSatisfy` (\x -> length x >= 1)