Rename _wrap headers to _internal

Makes it clearer that the user is not supposed to include them
diff --git a/include/mbedtls/md.h b/include/mbedtls/md.h
index f72dba1..3164788 100644
--- a/include/mbedtls/md.h
+++ b/include/mbedtls/md.h
@@ -65,7 +65,7 @@
 #endif
 
 /**
- * Opaque struct defined in md_wrap.h
+ * Opaque struct defined in md_internal.h
  */
 typedef struct mbedtls_md_info_t mbedtls_md_info_t;