Use consistent directory for cloning repositories

... with submodules.

lib/SlackBuilder/Trans.hs

@@ -6,14 +6,16 @@
 module SlackBuilder.Trans
     ( SlackBuilderException(..)
     , SlackBuilderT(..)
+    , relativeToRepository
     ) where
 
-import Control.Monad.Trans.Reader (ReaderT(..))
+import Control.Monad.Trans.Reader (ReaderT(..), asks)
 import Data.Text (Text)
 import SlackBuilder.Config
 import Control.Monad.IO.Class (MonadIO(..))
 import Control.Monad.Catch (MonadCatch(..), MonadThrow(..))
 import Control.Exception (Exception(..))
+import System.FilePath ((</>))
 
 newtype SlackBuilderException = UpdaterNotFound Text
     deriving Show
@@ -24,6 +26,10 @@ newtype SlackBuilderT a = SlackBuilderT
     { runSlackBuilderT :: ReaderT Settings IO a
     }
 
+relativeToRepository :: FilePath -> SlackBuilderT FilePath
+relativeToRepository filePath =
+    (</> filePath) <$> SlackBuilderT (asks repository)
+
 instance Functor SlackBuilderT
   where
     fmap f (SlackBuilderT slackBuilderT) = SlackBuilderT $ f <$> slackBuilderT