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)
```

1 files changed, 2 insertions, 0 deletions

diff --git a/lib/options.nix b/lib/options.nix
index d46dec6e41ca..a16a980398d6 100644
--- a/lib/options.nix
+++ b/lib/options.nix
@@ -48,6 +48,8 @@ rec {
     visible ? null,
     # Whether the option can be set only once
     readOnly ? null,
+    # Deprecated, used by types.optionSet.
+    options ? null
     } @ attrs:
     attrs // { _type = "option"; };