Revert "Remove which -> type -P alias."

This reverts commit e8e8164f34. I
misread the original commit as adding the "which" package, but it only
adds it to base.nix. So then the original motivation (making it work
in subshells) doesn't hold. Note that we already have some convenience
aliases that don't work in subshells either (such as "ll").

nixos/modules/profiles/base.nix

@@ -17,7 +17,6 @@
     pkgs.ddrescue
     pkgs.ccrypt
     pkgs.cryptsetup # needed for dm-crypt volumes
-    pkgs.which # 88K size
 
     # Some networking tools.
     pkgs.fuse

nixos/modules/programs/bash/bash.nix

@@ -56,7 +56,7 @@ in
       */
 
       shellAliases = mkOption {
-        default = config.environment.shellAliases;
+        default = config.environment.shellAliases // { which = "type -P"; };
         description = ''
           Set of aliases for bash shell. See <option>environment.shellAliases</option>
           for an option format description.