fix(parser): Don't panic with conflicts and groups

With #5298, I had overlooked that `matcher.arg_ids()` includes
`ArgGroup`s.  I had assumed I could always find a present `id` among
`Arg`s and `unwrap`ed.

I skipped a test for this because the use case is a bit strange that the
long term value for the test would likely be low.
If/when we add derive support for `args_conflicts_with_subcommands`, it
will then cover this case.

Fixes #5304

clap_builder/src/parser/parser.rs

@@ -514,7 +514,7 @@ impl<'cmd> Parser<'cmd> {
                     arg_os.display().to_string(),
                     matcher
                         .arg_ids()
-                        .map(|id| self.cmd.find(id).unwrap().to_string())
+                        .filter_map(|id| self.cmd.find(id).map(|a| a.to_string()))
                         .collect(),
                     Usage::new(self.cmd).create_usage_with_title(&[]),
                 );