aliases for package sets

[davidak]

Issue description

Package sets typically do not have aliases.

said @FRidh in #116475 (comment)

it would be good to have them, for example when a package is removed

the question is how to organize it.

it would add a huge mess to add an aliases file in addition to every package set in pkgs/top-level/

so i suggest to add a directory aliases in pkgs/top-level/

what do you think?

[dotlambda]

relevant: #116367