patch 8.2.3174: Vim9: "legacy undo" finds "undo" variable

Problem:    Vim9: "legacy undo" finds "undo" variable.
Solution:   Do not pass lookup function to find_ex_command(). (closes #8563)

Author: brammool

src/testdir/test_vim9_assign.vim

@@ -1632,6 +1632,13 @@ def Test_script_local_in_legacy()
     let s:legvar = 'one'
   END
   CheckScriptFailure(lines, 'E476:', 1)
+
+  edit! Xfile
+  lines =<< trim END
+      var edit: bool
+      legacy edit
+  END
+  CheckDefAndScriptSuccess(lines)
 enddef
 
 def Test_var_type_check()

src/version.c

@@ -755,6 +755,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    3174,
 /**/
     3173,
 /**/

src/vim9compile.c

@@ -9510,7 +9510,8 @@ compile_def_function(
 		}
 	    }
 	}
-	p = find_ex_command(&ea, NULL, starts_with_colon
+	p = find_ex_command(&ea, NULL,
+		starts_with_colon || (local_cmdmod.cmod_flags & CMOD_LEGACY)
 						  ? NULL : item_exists, &cctx);
 
 	if (p == NULL)