Create dex subdirectory

Move all the DexFile related source to a common subdirectory dex/ of
runtime.

Bug: 71361973
Test: make -j 50 test-art-host
Change-Id: I59e984ed660b93e0776556308be3d653722f5223
diff --git a/runtime/method_reference.h b/runtime/method_reference.h
index 31f3b8e..50b6d6e 100644
--- a/runtime/method_reference.h
+++ b/runtime/method_reference.h
@@ -19,8 +19,8 @@
 
 #include <stdint.h>
 #include <string>
-#include "dex_file.h"
-#include "dex_file_reference.h"
+#include "dex/dex_file.h"
+#include "dex/dex_file_reference.h"
 
 namespace art {