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)
diff --git a/src/vim9script.c b/src/vim9script.c
index a63f2f7..cd9ff92 100644
--- a/src/vim9script.c
+++ b/src/vim9script.c
@@ -415,6 +415,9 @@
 	si = SCRIPT_ITEM(*sid);
 	si->sn_import_autoload = TRUE;
 
+	if (si->sn_autoload_prefix == NULL)
+	    si->sn_autoload_prefix = get_autoload_prefix(si);
+
 	// with testing override: load autoload script right away
 	if (!override_autoload || si->sn_state != SN_STATE_NOT_LOADED)
 	    return OK;