vim-patch:9.0.1330: handling new value of an option has a long "else if" chain

Problem:    Handling new value of an option has a long "else if" chain.
Solution:   Use a function pointer. (Yegappan Lakshmanan, closes vim/vim#12015)

af93691b53

src/nvim/runtime.c

@@ -780,9 +780,10 @@ RuntimeSearchPath runtime_search_path_build(void)
   return search_path;
 }
 
-void runtime_search_path_invalidate(void)
+const char *did_set_runtimepackpath(optset_T *args)
 {
   runtime_search_path_valid = false;
+  return NULL;
 }
 
 void runtime_search_path_free(RuntimeSearchPath path)