Make core/ includes absolute, remove subfolders from include path

This allows more consistency in the manner we include core headers,
where previously there would be a mix of absolute, relative and
include path-dependent includes.

audio_driver_media_kit.cpp

@@ -32,7 +32,7 @@
 
 #ifdef MEDIA_KIT_ENABLED
 
-#include "project_settings.h"
+#include "core/project_settings.h"
 
 int32_t *AudioDriverMediaKit::samples_in = NULL;