The previous approach to loading config files was to ask WP for the
directory and then search those for the config files. This patch changes the
approach - a caller now asks WP to search for a specific config file or
iterate over a config file directory.
This allows us to implement a directory lookup order, i.e.
"wireplumber.conf" may be in XDG_CONFIG_DIR, /etc/,
/usr/share and the first one found is used.
For configuration directories, the new method iterates over all matching
entries (files + directories) and invokes a callback for each entry.
This enables distributions to ship default files in /usr/share/wireplumber
but have admins and users override them on a local basis. For lua scripts in
particular, overriding a distribution-provided file with an empty file
effectively disables it, adding a file adds it in the right sort order.
From the g_file_test documentation:
"For example, (G_FILE_TEST_EXISTS | G_FILE_TEST_IS_DIR) will return TRUE if
the file exists; the check whether it's a directory doesn't matter since the
existence test is TRUE. With the current set of available tests, there's no
point passing in more than one test at a time."
