lang/haskell: add intero support #158

To use dante instead, change `haskell` to `(haskell +dante)` in
~/.emacs.d/init.el.

modules/lang/haskell/packages.el

@@ -2,7 +2,13 @@
 ;;; lang/haskell/packages.el
 
 (package! haskell-mode)
-(package! dante)
-
 (when (featurep! :completion company)
   (package! company-ghc))
+
+;;
+(cond ((featurep! +dante)
+       (package! dante))
+      (t
+       (package! intero)
+       (package! hindent)))
+