I'm not really sure which one of types.lines or types.str that fit
better, but I'm going for types.lines because it behaves more like the
current type (i.e. have the ability to merge).
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.
