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/dictionary.cpp

@@ -30,9 +30,9 @@
 
 #include "dictionary.h"
 
-#include "ordered_hash_map.h"
-#include "safe_refcount.h"
-#include "variant.h"
+#include "core/ordered_hash_map.h"
+#include "core/safe_refcount.h"
+#include "core/variant.h"
 
 struct DictionaryPrivate {