Add a new private header file, wpbuildbasedirs.h, that contains the
build-time base directories passed directly from meson, without
the "wireplumber" suffix.
Use this to set the WP_BASE_DIRS_BUILD_* and adjust the code to honor
the SUBDIR_WIREPLUMBER flag.
This removes the previous hardcoding of this suffix and allows the
functions to be useful to other projects that use libwireplumber
and want to use this code to locate their data & config.
This allows the files iterator to lookup in a specific directory
when the given path is absolute, similar to how pipewire behaves
for configuration files (e.g. /foo/bar/wireplumber.conf must
include /foo/bar/wireplumber.conf.d/*.conf).
This also allows improving the wp_base_dirs_find_file() structure to
avoid duplicated code and add a debug message easily.
This makes things more consistent and allows us also to add more
search paths in the future if necessary.
Also, stop using g_module_build_path() because it's deprecated and
build the path manually. This obviously is not portable to Windows/Mac
or other exotic platforms, but portability is not important at this
point. PipeWire is also not portable beyond Linux & BSD.
This adds support for the system-wide locations for configuration and
data files, as defined by the XDG Base Directory Specification.
In addition, it adds two flag groups, CONFIGURATION and DATA, to the
base-dirs system, so that we don't have to hard-code the combinations
of flags everywhere.
