Treat paths in submodule relative to directory of root `.justfile`

It'd be great to have an option to **treat all paths in a submodule relative to the directory containing the root `.justfile`**. I think I'd prefer a *per-module* flag for this instead of a global one.

This would allow me to let my submodules reside in other file locations (e.g. in a folder `.config/commands/`), while not having to issue a `cd {{justfile_directory()}}` or `cd ../../` at the beginning of every recipe.