Move substitute_path_component

Move substitute_path_component

This moves substitute_path_component out of utils.c.  I considered
making a new file for this (still could if someone wants that), but
since the only caller is in auto-load.c, I moved it there instead.

I've also moved the tests into auto-load.c as well.  This way
substitute_path_component can be static.

Approved-By: Simon Marchi <simon.marchi@efficios.com>