refactor: remove some unused includes (#19747)

- Remove autocmd.h from fileio.h
- Remove normal.h from main.h
- Move bufinfo_T from undo_defs.h to undo.c

src/nvim/api/private/helpers.c

@@ -21,7 +21,6 @@
 #include "nvim/ex_cmds_defs.h"
 #include "nvim/ex_eval.h"
 #include "nvim/extmark.h"
-#include "nvim/fileio.h"
 #include "nvim/highlight_group.h"
 #include "nvim/lib/kvec.h"
 #include "nvim/lua/executor.h"