Avoid Arg::id ambiguity in derive by naming it for Type::field instead of field

[epage]

Please complete the following tasks

• I have searched the discussions
• I have searched the open and rejected issues

Clap Version

4.1.0

Describe your use case

In clap v4, we switched the derive's implicit Arg::id to be the literal field name to avoid confusion over _ -> -. This can cause ambiguities across flattens, even if the longs are made unique.

Describe the solution you'd like

What if we qualified these fields by their type?

The biggest problem for Arg::id is discoverability. For naive cases (no flattening), which is more obvious, an unqualified name or a qualified name? For non-naive cases, I think the user could understand the use of qualified names.

Alternatives, if applicable

Do nothing.

Additional Context

#4699 has a related problem with variant structs in an ArgGroup. Since this is new design work, we've noted in #2621 to set the Arg::id according to Variant::field which inspired this issue.