derive(ArgEnum) should be distinct from derive(Clap)

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• I have searched the discussions
• I have searched the existing issues

Rust Version

rustc 1.53.0 (53cb7b09b 2021-06-17)

Clap Version

3be79b9

Minimal reproducible code

Naively just using the "magic" struct

#[derive(clap::Clap)]
enum Level {
  Info,
  Warn,
  Error,
}

Intentionally creating subcommand

#[derive(clap::Clap)]
enum Action {
  Add,
  Remove,
}

Steps to reproduce the bug with the above code

Can use arg in wrong case

#[derive(clap::Clap)]
struct Args {
  #[clap(subcommand)]
  level: Level,
}

Can use subcommand in wrong case

#[derive(clap::Clap)]
struct Args {
    #[clap(arg_enum)]
    action: Action,
}

Actual Behaviour

API can be used in the wrong way

Expected Behaviour

Compile errors

Additional Context

This is split out of #2005, see

• Use enum as subcommands in a subcommand #2005 (comment)
• Use enum as subcommands in a subcommand #2005 (comment)

Debug Output

No response