How can I register a package in an arbitrary package database with new style commands?

I have been using `cabal register --gen-pkg-config` to this end. But there is currently no `new-register` command at all. _(I am using `cabal-2.4.0.0`, built from commit `c7081a8b9`.)_

My case is that I have several `ghc` versions variously obtained via `stack` and `nix`, and I find it comfortable to compile individual, unpackaged Haskell drafts that, nevertheless, may have some non-trivial dependencies. So, it is handy to have a simple way of defining my own package databases.