Simplify auto_load_expand_dir_vars and remove substitute_path_component

Simplify auto_load_expand_dir_vars and remove substitute_path_component

This simplifies auto_load_expand_dir_vars to first split the string,
then do any needed substitutions.  This was suggested by Simon, and is
much simpler than the current approach.

Then this patch also removes substitute_path_component, as it is no
longer called.  This is nice because it helps with the long term goal
of removing utils.h.

Regression tested on x86-64 Fedora 36.