* Make the flags public and give them nicer names
* Pass down the flags from the caller, so the caller can now explicitly
ask for looking into specific directories
* Rename the methods
* Remove and inline the wp_get_xdg_config_dir() method, since it's only
used internally
* Refactor the lookup dirs ordering to get both WIREPLUMBER_*_DIR env
variables to replace all the other directories. Previously, we were looking
for scripts in WIREPLUMBER_DATA_DIR, but we were also looking in /etc at
the same time (with precedence, even), which could result in unexpected
behaviour. Now, if a WIREPLUMBER environment variable is specified,
we only look in there.