clean up the :mini_table build target

Remove circular dependencies that were bouncing back and forth between
msg_internal.h and mini_table/, including:

- splitting out each mini table subtype into its own header
- moving the non-reflection message code into message/
- moving the accessors from mini_table/ to message/

PiperOrigin-RevId: 489121042
diff --git a/upb/fuzz_test_util.h b/upb/fuzz_test_util.h
index 9127b17..10728fd 100644
--- a/upb/fuzz_test_util.h
+++ b/upb/fuzz_test_util.h
@@ -31,7 +31,8 @@
 #include <string>
 #include <vector>
 
-#include "upb/mini_table.h"
+#include "upb/extension_registry.h"
+#include "upb/mini_table/types.h"
 
 namespace upb {
 namespace fuzz {