Change the "x_mod.lua" files to folders

[sprunk]

Some files (customcmds.lua, unitdefs_post.lua etc) look for a _mod file so that mods don't have to override the vanilla file. But you can have a mod of a mod, or load two mods in parallel. Collections of mod content could become folders instead (so you'd have a Commands folder and everything there is a command, a DefsPost folder and everything is a processing bit, etc).