patch 8.2.4104: Vim9: lower casing the autoload prefix causes problems

Problem:    Vim9: lower casing the autoload prefix causes problems.
Solution:   Always store the prefix with case preserved.
diff --git a/src/testdir/test_vim9_import.vim b/src/testdir/test_vim9_import.vim
index 79e5728..6c6911a 100644
--- a/src/testdir/test_vim9_import.vim
+++ b/src/testdir/test_vim9_import.vim
@@ -1485,6 +1485,15 @@
   END
   CheckScriptSuccess(lines)
 
+  if !has('fname_case')
+    lines =<< trim END
+        vim9script
+        import autoload 'CaseSensitive.vim'
+        import autoload 'casesensitive.vim'
+    END
+    CheckScriptFailure(lines, 'E1262:')
+  endif
+
   delete('Xdir', 'rf')
   &rtp = save_rtp
 enddef