nix: Add a "dev" output

This gets rid of boehm-dev in the closure (as well as Nix's own
headers).

nixos/default.nix

···
   vmWithBootLoader = vmWithBootLoaderConfig.system.build.vm;
 
   # The following are used by nixos-rebuild.
-  nixFallback = pkgs.nixUnstable;
+  nixFallback = pkgs.nixUnstable.out;
 }

nixos/modules/services/misc/nix-daemon.nix

···
 
       package = mkOption {
         type = types.package;
-        default = pkgs.nix;
-        defaultText = "pkgs.nix";
+        default = pkgs.nix.out;
+        defaultText = "pkgs.nix.out";
         description = ''
           This option specifies the Nix package instance to use throughout the system.
         '';

pkgs/build-support/replace-dependency.nix

···
   drvName = drv:
     discard (substring 33 (stringLength (builtins.baseNameOf drv)) (builtins.baseNameOf drv));
 
-  rewriteHashes = drv: hashes: runCommand (drvName drv) { nixStore = "${nix}/bin/nix-store"; } ''
+  rewriteHashes = drv: hashes: runCommand (drvName drv) { nixStore = "${nix.out}/bin/nix-store"; } ''
     $nixStore --dump ${drv} | sed 's|${baseNameOf drv}|'$(basename $out)'|g' | sed -e ${
       concatStringsSep " -e " (mapAttrsToList (name: value:
         "'s|${baseNameOf name}|${baseNameOf value}|g'"

pkgs/development/ruby-modules/bundix/default.nix

···
     substituteInPlace $GEM_HOME/gems/${gemName}-${version}/lib/bundix.rb \
       --replace \
         "'nix-instantiate'" \
-        "'${nix}/bin/nix-instantiate'" \
+        "'${nix.out}/bin/nix-instantiate'" \
       --replace \
         "'nix-hash'" \
-        "'${nix}/bin/nix-hash'" \
+        "'${nix.out}/bin/nix-hash'" \
       --replace \
         "'nix-prefetch-url'" \
-        "'${nix}/bin/nix-prefetch-url'" \
+        "'${nix.out}/bin/nix-prefetch-url'" \
       --replace \
         "'nix-prefetch-git'" \
         "'${nix-prefetch-git}/bin/nix-prefetch-git'"

pkgs/tools/package-management/nix/default.nix

···
   common = { name, src }: stdenv.mkDerivation rec {
     inherit name src;
 
-    outputs = [ "out" "man" "doc" ];
+    outputs = [ "dev" "out" "man" "doc" ];
 
     nativeBuildInputs = [ perl pkgconfig ];