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.

drivers/wasapi/audio_driver_wasapi.cpp

@@ -32,8 +32,8 @@
 
 #include "audio_driver_wasapi.h"
 
-#include "os/os.h"
-#include "project_settings.h"
+#include "core/os/os.h"
+#include "core/project_settings.h"
 
 #include <functiondiscoverykeys.h>