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/math/a_star.cpp

@@ -29,9 +29,10 @@
 /*************************************************************************/
 
 #include "a_star.h"
-#include "geometry.h"
+
+#include "core/math/geometry.h"
+#include "core/script_language.h"
 #include "scene/scene_string_names.h"
-#include "script_language.h"
 
 int AStar::get_available_point_id() const {