haskellPackages.Agda: Split outputs to reduce closure size

After enabling a separate binary output for the `Agda` Haskell package,
the new `bin` output measures ~100MiB, compared to the ~4.5GiB before.
Using it in `agdaWithPackages` reduces the closure size of an Agda
installation from ~5GiB to ~3GiB. The remaining space is taken up
mostly by the GHC backend.

With this change, derivations depending on `haskellPackages.Agda`
directly need to pick the right (binary) output. This concerns in
particular `emacsPackages.agda2-mode`.

pkgs/applications/editors/emacs/elisp-packages/manual-packages/agda2-mode/default.nix

···
 { trivialBuild
 , haskellPackages
 }:
-
-trivialBuild rec {
+let
+  Agda = haskellPackages.Agda.bin;
+in
+trivialBuild {
   pname = "agda-mode";
-  version = haskellPackages.Agda.version;
+  version = Agda.version;
 
   dontUnpack = true;
 
   # already byte-compiled by Agda builder
   buildPhase = ''
-    agda=`${haskellPackages.Agda}/bin/agda-mode locate`
+    agda=`${Agda}/bin/agda-mode locate`
     cp `dirname $agda`/*.el* .
   '';
 
   meta = {
-    inherit (haskellPackages.Agda.meta) homepage license;
+    inherit (Agda.meta) homepage license;
     description = "Agda2-mode for Emacs extracted from Agda package";
     longDescription = ''
       Wrapper packages that liberates init.el from `agda-mode locate` magic.

pkgs/build-support/agda/default.nix

···
         allPackages = withPackages (filter self.lib.isUnbrokenAgdaPackage (attrValues self));
       };
     };
-    inherit (Agda) meta;
+    # Agda is a split package with multiple outputs; do not inherit them here.
+    meta = removeAttrs Agda.meta [ "outputsToInstall" ];
   } ''
     mkdir -p $out/bin
-    makeWrapper ${Agda}/bin/agda $out/bin/agda \
+    makeWrapper ${Agda.bin}/bin/agda $out/bin/agda \
       --add-flags "--with-compiler=${ghc}/bin/ghc" \
       --add-flags "--library-file=${library-file}"
-    ln -s ${Agda}/bin/agda-mode $out/bin/agda-mode
+    ln -s ${Agda.bin}/bin/agda-mode $out/bin/agda-mode
     '';
 
   withPackages = arg: if isAttrs arg then withPackages' arg else withPackages' { pkgs = arg; };

pkgs/development/haskell-modules/configuration-nix.nix

···
     # very useful.
     # Flag added in Agda 2.6.4.1, was always enabled before
     (enableCabalFlag "debug")
+    # Split outputs to reduce closure size
+    enableSeparateBinOutput
   ];
 
   # ats-format uses cli-setup in Setup.hs which is quite happy to write