Following https://github.com/jj-vcs/jj/pull/6300, Jujutsu has deprecated
support for configuration files in `~/Library/Application Support` for
darwin. The XDG-standard configuration location can be used instead, for
all platforms.
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.
