Instead of having to manually stub packages that should not be
downloaded we instead automatically stub all packages (except a small
list of whitelisted ones). Tests can re-introduce the real package by
using the `realPkgs` module argument.
NixOS/nixpkgs@03310df843 disabled flake
support by default, so we now need to build a custom package and use it
if the user wants to `use flake` successfully. This should fix#2087.
