Modular X doesn't have xf86Parser.h anywhere (at least not that I could find). This is used by pyxf86config and our config tools for writing out X configs.
Upstream modular X does not provide any tarball for libxf86config yet. Can you report this upstream, so we can track it?
Fixed in xorg-x11-server-1.0.0-2 The headers are now in the sdk subpackage.