Here's another crack at it using the options subsystems this time. (Thanks for the suggestions Sean and Manuel!)
The original summary:
In using modularize to check a large group of platform headers for modules-readiness, I found that a few headers had dependencies, such that they required other headers to be included first to avoid compile errors on missing definitions.
This patch adds support to modularize to allow specifying depended-on headers in the header file list input to modularize, i.e.
header.h: dependency1.h dependency2.h