wp: make the config file lookup methods more generic
- 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.