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/main/http_request.h

@@ -31,10 +31,10 @@
 #ifndef HTTPREQUEST_H
 #define HTTPREQUEST_H
 
-#include "io/http_client.h"
+#include "core/io/http_client.h"
+#include "core/os/file_access.h"
+#include "core/os/thread.h"
 #include "node.h"
-#include "os/file_access.h"
-#include "os/thread.h"
 
 class HTTPRequest : public Node {