Handle deprecated package versions

I noticed that the latest nightly snapshots, e.g. [`2022-04-24`](https://www.stackage.org/nightly-2022-04-24) and [`2022-04-23`](https://www.stackage.org/nightly-2022-04-23), contain [`unordered-containers-0.2.19.0`](https://hackage.haskell.org/package/unordered-containers-0.2.19.0), despite it being deprecated on Hackage since 2022-04-17 due to https://github.com/haskell-unordered-containers/unordered-containers/issues/420, a correctness bug.

I wonder whether Stackage should possibly include checks to ensure that the snapshots don't include deprecated packages?

Alternatively, should package maintainers take additional steps when deprecating packages, to prevent Stackage users from using them? I suspect I should have created an impossible dependency constraint like `base < 0` in a Hackage revision. (EDIT: I have done this now. See https://hackage.haskell.org/package/unordered-containers-0.2.19.0/revisions/)