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/CabalApiSpec.hs

@@ -9,6 +9,8 @@ import Language.Haskell.GhcMod.CabalApi
 import Language.Haskell.GhcMod.Cradle
 import Language.Haskell.GhcMod.Types
 import Test.Hspec
+import System.Directory
+import System.FilePath
 
 import Dir
 
@@ -20,6 +22,7 @@ spec = do
 
     describe "getCompilerOptions" $ do
         it "gets necessary CompilerOptions" $ do
+            cwd <- getCurrentDirectory
             withDirectory "test/data/subdir1/subdir2" $ \dir -> do
                 cradle <- findCradle
                 pkgDesc <- parseCabalFile $ fromJust $ cradleCabalFile cradle
@@ -28,7 +31,7 @@ spec = do
                         ghcOptions  = ghcOptions res
                       , includeDirs = map (toRelativeDir dir) (includeDirs res)
                       }
-                res' `shouldBe` CompilerOptions {ghcOptions = ["-no-user-package-db","-package-db","/home/me/work/ghc-mod/test/data/.cabal-sandbox/i386-osx-ghc-7.6.3-packages.conf.d","-XHaskell98"], includeDirs = ["test/data","test/data/dist/build","test/data/dist/build/autogen","test/data/subdir1/subdir2","test/data/test"], depPackages = [("Cabal", Nothing), ("base", Nothing) , ("template-haskell", Nothing)]}
+                res' `shouldBe` CompilerOptions {ghcOptions = ["-global-package-db", "-no-user-package-db","-package-db",cwd </> "test/data/.cabal-sandbox/i386-osx-ghc-7.6.3-packages.conf.d","-XHaskell98"], includeDirs = ["test/data","test/data/dist/build","test/data/dist/build/autogen","test/data/subdir1/subdir2","test/data/test"], depPackages = [("Cabal", Nothing), ("base", Nothing) , ("template-haskell", Nothing)]}
 
     describe "cabalDependPackages" $ do
         it "extracts dependent packages" $ do