nixos manuals: bring back package references

This reverts most of 89e983786a, as those references are sanitized now.
Fixes #10039, at least most of it.

The `sane` case wasn't fixed, as it calls a *function* in pkgs to get
the default value.

nixos/modules/programs/ssh.nix

@@ -36,6 +36,7 @@ in
 
       askPassword = mkOption {
         type = types.str;
+        default = "${pkgs.x11_ssh_askpass}/libexec/x11-ssh-askpass";
         description = ''Program used by SSH to ask for passwords.'';
       };
 
@@ -222,7 +223,5 @@ in
         export SSH_ASKPASS=${askPassword}
       '';
 
-    programs.ssh.askPassword = mkDefault "${pkgs.x11_ssh_askpass}/libexec/x11-ssh-askpass";
-
   };
 }