patch 8.2.4257: Vim9: finding global function without g: prefix inconsistent

Problem:    Vim9: finding global function without g: prefix but not finding
            global variable is inconsistent.
Solution:   Require using g: for a global function.  Change the vim9.vim
            script into a Vim9 script with exports.  Fix that import in legacy
            script does not work.
diff --git a/src/eval.c b/src/eval.c
index 86f103b..c235788 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -963,7 +963,7 @@
     if (lp->ll_name == NULL)
 	return p;
 
-    if (*p == '.' && in_vim9script())
+    if (*p == '.')
     {
 	imported_T *import = find_imported(lp->ll_name, p - lp->ll_name,
 								   TRUE, NULL);