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.

src/scriptfile.c

@@ -2156,11 +2156,7 @@ get_autoload_prefix(scriptitem_T *si)
 
     if (p == NULL)
 	return NULL;
-#ifdef CASE_INSENSITIVE_FILENAME
-    prefix = strlow_save(p);
-#else
     prefix = vim_strsave(p);
-#endif
     if (prefix == NULL)
 	return NULL;
 

src/testdir/test_vim9_import.vim

@@ -1485,6 +1485,15 @@ def Test_vim9_autoload_case_sensitive()
   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

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    4104,
 /**/
     4103,
 /**/