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.

Changed files
+2 -3
pkgs
development
compilers
scala
+2 -3
pkgs/development/compilers/scala/default.nix
···
'';
inherit (bare) meta;
-
}
-
// {
-
inherit bare;
+
+
passthru = { inherit bare; };
}