put clap_complete behind a feature flag instead of a separate crate

[contagnas]

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• I have searched the open and rejected issues

Clap Version

master

Describe your use case

Currently clap_complete is shipped as a separate crate. This hurts discoverability (at least for me), and means users need to keep the dependency versions in sync.

Describe the solution you'd like

Put clap_complete behind a feature flag, like derive

Alternatives, if applicable

No response

Additional Context

No response

[epage]

clap used to include completions (see App::gen_completions and it was intentionally split out as part of the v3 effort.

Features have several usability problems in Rust though some are made up for with some unstable rustdoc features we use with docs.rs.

In my mind, the better route for us to be going is to leverage rust-lang/rfcs#3243 when its approved.