No way to specify other-modules for custom Setup section

I don't know if this is affecting anyone, but there isn't any way to specify `other-modules` for a Setup script (e.g., if it spans multiple modules). A reasonable workaround to make sure the script does get added to the sdist tarball is to put it in extra-source-files, but if we ever switch to doing per-component recompilation avoidance, the lack of accurate dependency information on Setup files will prevent us from knowing when to rebuild a Setup script when you edit NOT the Setup.hs file itself.

I don't plan on fixing this unless someone actually runs into it.