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/unix/thread_posix.cpp

@@ -29,7 +29,7 @@
 /*************************************************************************/
 
 #include "thread_posix.h"
-#include "script_language.h"
+#include "core/script_language.h"
 
 #if (defined(UNIX_ENABLED) || defined(PTHREAD_ENABLED)) && !defined(NO_THREADS)
 
@@ -37,8 +37,8 @@
 #include <pthread_np.h>
 #endif
 
+#include "core/os/memory.h"
 #include "core/safe_refcount.h"
-#include "os/memory.h"
 
 static pthread_key_t _create_thread_id_key() {
 	pthread_key_t key;