Implement --keep

lib/GHCup/Types.hs

@@ -142,10 +142,17 @@ data URLSource = GHCupURL
 data Settings = Settings
   { cache    :: Bool
   , noVerify :: Bool
+  , keepDirs :: KeepDirs
   }
   deriving Show
 
 
+data KeepDirs = Always
+              | Errors
+              | Never
+  deriving (Eq, Show, Ord)
+
+
 data DebugInfo = DebugInfo
   { diBaseDir  :: Path Abs
   , diBinDir   :: Path Abs