base-dirs: add support for finding modules and remove wp_get_module_dir()

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.

lib/wp/wp.h

@@ -76,9 +76,6 @@ const char * wp_get_library_version (void);
 WP_API
 const char * wp_get_library_api_version (void);
 
-WP_API
-const gchar * wp_get_module_dir (void);
-
 G_END_DECLS
 
 #endif