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.
This enables to override which plugins to include with nginx by allowing to change
nginx package. I also removed webdav option in nginx nixos module, because you
can now specify this by overriding nginx package.
