packages-config.nix: make sure all GHCs are added to packages.json

Currently only the default GHC (8.10.7) is added to packages.json,
leading e. g. repology to believe we don't ship an up to date GHC which
is inaccurate.

pkgs/top-level/packages-config.nix

@@ -31,6 +31,13 @@
       zeroadPackages
     ;
 
+    # Make sure haskell.compiler is included, so alternative GHC versions show up,
+    # but don't add haskell.packages.* since they contain the same packages (at
+    # least by name) as haskellPackages.
+    haskell = super.haskell // {
+      compiler = recurseIntoAttrs super.haskell.compiler;
+    };
+
     # This is an alias which we disallow by default; explicitly allow it
     emacs27Packages = emacs27.pkgs;
   };