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.

scene/2d/line_builder.h

@@ -31,10 +31,10 @@
 #ifndef LINE_BUILDER_H
 #define LINE_BUILDER_H
 
-#include "color.h"
+#include "core/color.h"
+#include "core/math/vector2.h"
 #include "line_2d.h"
 #include "scene/resources/color_ramp.h"
-#include "vector2.h"
 
 class LineBuilder {
 public: