HAProxy fails to start with the default 'config'. Better disable it and
assert that the user provides a suitable 'config'. (AFAICS, there cannot
really be a default config file for HAProxy.)
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.
