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/buffer_defs.h

@@ -30,7 +30,7 @@ typedef struct {
 #include "nvim/option_defs.h"
 // for jump list and tag stack sizes in a buffer and mark types
 #include "nvim/mark_defs.h"
-// for u_header_T; needs buf_T.
+// for u_header_T
 #include "nvim/undo_defs.h"
 // for hashtab_T
 #include "nvim/hashtab.h"