This option makes the coupling between lighttpd and its sub-services
more "loose".
While the option is a list, its purpose is to provide a "set" of needed
modules to load for lighttpd to function correctly with its config. The
NixOS lighttpd module ensures that lighttpd modules are loaded no more
than once (because lighttpd dislikes that), and in the correct order.
Also add an assertion that all modules listed in .enableModules are
valid.
Setting "services.lighttpd.gitweb.enable" to true doesn't enable the
required lighttpd modules to actually make it work. The problem is that
"or" and "||" don't mean the same thing: "or" falls back to the second
operand if the first is not defined, whereas "||" is the normal logical
operator. When cfg.cgit.enable is defined, as false, the expressions
don't have the desired effect.
[Bjørn: modify commit message]
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.
