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_plugin_settings.cpp

@@ -30,11 +30,11 @@
 
 #include "editor_plugin_settings.h"
 
+#include "core/io/config_file.h"
+#include "core/os/file_access.h"
+#include "core/os/main_loop.h"
+#include "core/project_settings.h"
 #include "editor_node.h"
-#include "io/config_file.h"
-#include "os/file_access.h"
-#include "os/main_loop.h"
-#include "project_settings.h"
 #include "scene/gui/margin_container.h"
 
 void EditorPluginSettings::_notification(int p_what) {