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.
The configuration file format of borgmatic has changed in version
1.8.0:
https://projects.torsion.org/borgmatic-collective/borgmatic/src/branch/main/NEWS
This commit makes Home Manager generate borgmatic's configuration file
using the new format.
Even though the NEWS file indicates that old configuration files are
compatible, this is not 100% the case: empty sections work fine in old
borgmatic but stop working in new ones. I've reported the bug upstream
by email as I couldn't create an account on the forge.
