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.
This will reduce the system closure size by about 200MB under NixOS by
sharing the glibcLocales package.
When home-manager is installed on Linux without the NixOS module, all
glibc locales are installed, as before.
Resolves: #2333
