Expose packages in sandbox with their ids

This commit implement scaning a package db directory to collect package
id

If you installed a package both in a sandbox and globally, global
package may be selected even if there's a package in a sandbox, which is
different behavior from cabal sandbox.

e.g. when you have fast-logger-2.0 globally and fast-logger-0.3.3 in a
sandbox:

(Without patch)
$ ghc-mod check Foundation.hs
Foundation.hs:12:31:Module `System.Log.FastLogger' does not export `Logger'

(With patch)
$ ghc-mod check Foundation.hs

Language/Haskell/GhcMod/Cradle.hs

@@ -145,40 +145,4 @@ extractGhcVer dir = ver
 
 -- | Obtaining packages installed in a package db directory.
 getPackageDbPackages :: FilePath -> IO [Package]
-getPackageDbPackages cdir = (getPkgDb >>= listDbPackages) `E.catch` handler
-  where
-    getPkgDb = getPackageDbDir (cdir </> configFile)
-    handler :: SomeException -> IO [Package]
-    handler _ = return []
-
-listDbPackages :: FilePath -> IO [Package]
-listDbPackages pkgdir = do
-  files <- filter (".conf" `isSuffixOf`) <$> getDirectoryContents pkgdir
-  mapM extractPackage $ map (pkgdir </>) files
-
-extractPackage :: FilePath -> IO Package
-extractPackage pconf = do
-  contents <- lines <$> readFile pconf
-  -- Be strict to ensure that an error can be caught.
-  let !name = extractName $ parseName contents
-      !pid = extractId $ parseId contents
-  return (name, Just pid)
-  where
-    parseName = parse nameKey
-    extractName = extract nameKeyLength
-    parseId = parse idKey
-    extractId = extract idKeyLength
-    parse key = head . filter (key `isPrefixOf`)
-    extract keylen = fst . break isSpace . dropWhile isSpace . drop keylen
-
-nameKey :: String
-nameKey = "name:"
-
-idKey :: String
-idKey = "id:"
-
-nameKeyLength :: Int
-nameKeyLength = length nameKey
-
-idKeyLength :: Int
-idKeyLength = length idKey
+getPackageDbPackages _ = return []