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.

editor/editor_log.cpp

@@ -31,10 +31,10 @@
 #include "editor_log.h"
 
 #include "core/os/keyboard.h"
+#include "core/version.h"
 #include "editor_node.h"
 #include "scene/gui/center_container.h"
 #include "scene/resources/dynamic_font.h"
-#include "version.h"
 
 void EditorLog::_error_handler(void *p_self, const char *p_func, const char *p_file, int p_line, const char *p_error, const char *p_errorexp, ErrorHandlerType p_type) {