Option defaults should not refer to store paths, because they cause
the manual to be rebuilt gratuitously. It's especially bad to refer to
a highly variable path like a computed configuration file.
Using pkgs.lib on the spine of module evaluation is problematic
because the pkgs argument depends on the result of module
evaluation. To prevent an infinite recursion, pkgs and some of the
modules are evaluated twice, which is inefficient. Using ‘with lib’
prevents this problem.
