system-path: allow other modules to provide setup fragments

nixos/modules/config/system-path.nix

···
         description = "List of additional package outputs to be symlinked into <filename>/run/current-system/sw</filename>.";
       };
 
+      extraSetup = mkOption {
+        type = types.lines;
+        default = [ ];
+        description = "Shell fragments to be run after the system environment has been created. This should only be used for things that need to modify the internals of the environment, e.g. generating MIME caches. The environment being built can be accessed at $out.";
+      };
+
     };
 
     system = {
···
           if [ -x $out/bin/glib-compile-schemas -a -w $out/share/glib-2.0/schemas ]; then
               $out/bin/glib-compile-schemas $out/share/glib-2.0/schemas
           fi
-
-          if [ -x $out/bin/install-info -a -w $out/share/info ]; then
-            shopt -s nullglob
-            for i in $out/share/info/*.info $out/share/info/*.info.gz; do
-                $out/bin/install-info $i $out/share/info/dir
-            done
-          fi
-        '' ++ optionalString config.xdg.mime.enable ''
-          XDG_DATA_DIRS=$out/share ${pkgs.shared-mime-info}/bin/update-mime-database -V $out/share/mime > /dev/null
-
-          ${pkgs.desktop-file-utils}/bin/update-desktop-database $out/share/applications
+          
+          ${config.environment.extraSetup}
         '';
     };
 

nixos/modules/config/xdg/mime.nix

···
       # this package also installs some useful data, as well as its utilities 
       pkgs.shared-mime-info 
     ];
+
+    environment.extraSetup = ''
+      if [ -w $out/share/mime ]; then
+        XDG_DATA_DIRS=$out/share ${pkgs.shared-mime-info}/bin/update-mime-database -V $out/share/mime > /dev/null
+      fi
+
+      if [ -w $out/share/applications ]; then
+        ${pkgs.desktop-file-utils}/bin/update-desktop-database $out/share/applications
+      fi
+    '';
   };
 
 }

nixos/modules/misc/documentation.nix

···
       environment.systemPackages = [ pkgs.texinfoInteractive ];
       environment.pathsToLink = [ "/share/info" ];
       environment.extraOutputsToInstall = [ "info" ] ++ optional cfg.dev.enable "devinfo";
+      environment.extraSetup = ''
+        if [ -w $out/share/info ]; then
+          shopt -s nullglob
+          for i in $out/share/info/*.info $out/share/info/*.info.gz; do
+              ${pkgs.texinfo}/bin/install-info $i $out/share/info/dir
+          done
+        fi
+      '';
     })
 
     (mkIf cfg.doc.enable {