How to build a file which depends on the actual contents of store items?

[宋文武]

when look at our xorg service, the first thing I want to do is remove
the hard-coded modules and fonts from the xserver.conf.
the attach show what I intend to do, but I don't know how to
feed `mixed-text-file` the desired arguments now.

for each module, a line of `ModulePath "$package/lib/xorg/modules/"`
(or more specified one like `lib/xorg/modules/drivers`?  I haven't test)
need to be added to the Files Section.  Since package is an object,
I can't use string-append to compose them as one argument to
mixed-text-file,  do I need a macro?

for fonts, it's more difficult (and on topic), because it does need to
know the actual contents (the directories contains fonts.dir).

see NixOS, it did the same thing by run shell commands on store items 
:-)
https://github.com/NixOS/nixpkgs/blob/master/nixos/modules/services/x11/xserver.nix#L74-L105

How I can do it?

wip-xorg.scm
Description: Text Data