Ability to surgically remove some packages from Nix global store

Right now, we don't have GC (#3333) so the only way to clean up the store is to just delete it wholesale.

Once, I tried deleting a specific package by unregistering it from the database. But I guess someone else was depending on it because cabal new-build choked to death and I couldn't recover without deleting the store.

So it would seem like it would be helpful and orthogonal to GC to allow users to delete specific packages from the store, recursively removing anything that depended on them. (Maybe not that useful? I don't know.)