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/compiler/optimizing/find_loops_test.cc b/compiler/optimizing/find_loops_test.cc
index c917528..b799fb4 100644
--- a/compiler/optimizing/find_loops_test.cc
+++ b/compiler/optimizing/find_loops_test.cc
@@ -16,8 +16,8 @@
 
 #include "base/arena_allocator.h"
 #include "builder.h"
-#include "dex_file.h"
-#include "dex_instruction.h"
+#include "dex/dex_file.h"
+#include "dex/dex_instruction.h"
 #include "nodes.h"
 #include "optimizing_unit_test.h"
 #include "pretty_printer.h"