module system: revert "remove types.optionSet", just deprecate (#56857)

The explicit remove helped to uncover some hidden uses of `optionSet`
in NixOps. However it makes life harder for end-users of NixOps - it will
be impossible to deploy 19.03 systems with old NixOps, but there is no
new release of NixOps with `optionSet` fixes.

Also, "deprecation" process isn't well defined. Even that `optionSet` was
declared "deprecated" for many years, it was never announced. Hence, I
leave "deprecation" announce. Then, 3 releases after announce,
we can announce removal of this feature.

This type has to be removed, not `throw`-ed in runtime, because it makes
some perfectly fine code to fail. For example:
```
$ nix-instantiate --eval -E '(import <nixpkgs/lib>).types' --strict
trace: `types.list` is deprecated; use `types.listOf` instead
error: types.optionSet is deprecated; use types.submodule instead
(use '--show-trace' to show detailed location information)
```

lib/types.nix

@@ -469,8 +469,10 @@ rec {
     # Obsolete alternative to configOf.  It takes its option
     # declarations from the ‘options’ attribute of containing option
     # declaration.
-    optionSet = builtins.throw "types.optionSet is deprecated; use types.submodule instead" "optionSet";
-
+    optionSet = mkOptionType {
+      name = builtins.trace "types.optionSet is deprecated; use types.submodule instead" "optionSet";
+      description = "option set";
+    };
     # Augment the given type with an additional type check function.
     addCheck = elemType: check: elemType // { check = x: elemType.check x && check x; };