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.

core/os/input_event.cpp

@@ -30,8 +30,8 @@
 
 #include "input_event.h"
 
-#include "input_map.h"
-#include "os/keyboard.h"
+#include "core/input_map.h"
+#include "core/os/keyboard.h"
 
 void InputEvent::set_device(int p_device) {
 	device = p_device;