scala: refactor bare to passthru

The `.bare` attribute is probably only supposed to be used inside
nixpkgs, i.e. for `scala-next`. Still, this link breaks in some obscure
override scenarios, so let's improve that with a passthru.

Also: Any attribute concatenation onto the result of mkDerivation will
break some overrideability.

pkgs/development/compilers/scala/default.nix

@@ -32,7 +32,6 @@ stdenv.mkDerivation {
   '';
 
   inherit (bare) meta;
-}
-// {
-  inherit bare;
+
+  passthru = { inherit bare; };
 }