patch 8.2.4656: Vim9: can't use item from "import autoload" with autoload dir

Problem:    Vim9: can't use items from "import autoload" with autoload
            directory name.
Solution:   Let sn_autoload_prefix overrule sn_import_autoload.
            (closes #10054)

src/structs.h

@@ -1868,6 +1868,7 @@ typedef struct
     char_u	*sn_autoload_prefix;
 
     // TRUE for a script used with "import autoload './dirname/script.vim'"
+    // For "../autoload/script.vim" sn_autoload_prefix is also set.
     int		sn_import_autoload;
 
 # ifdef FEAT_PROFILE