refactor: run IWYU on entire repo

Reference: https://github.com/neovim/neovim/issues/6371.

src/nvim/buffer.h

@@ -6,13 +6,10 @@
 
 #include "nvim/buffer_defs.h"  // IWYU pragma: export
 #include "nvim/eval/typval.h"
-#include "nvim/eval/typval_defs.h"
-#include "nvim/ex_cmds_defs.h"
 #include "nvim/func_attr.h"
 #include "nvim/macros_defs.h"
 #include "nvim/memline.h"
-#include "nvim/memline_defs.h"
-#include "nvim/pos_defs.h"
+#include "nvim/types_defs.h"
 
 /// Values for buflist_getfile()
 enum getf_values {