fix(complete): Prefer subcommand names over aliases

[cyphercodes]

Summary

• keep dynamic subcommand completion candidates in declaration/display order before id de-duplication
• prefer a subcommand's primary name over a matching visible alias when only one candidate is shown
• add a regression test for install / visible alias i

Fixes #6317

Testing

• cargo fmt --all --check
• cargo test -p clap_complete --features unstable-dynamic --test testsuite engine::
• cargo test -p clap_complete --features unstable-dynamic --test testsuite
• cargo test -p clap_complete --features unstable-dynamic

[epage]

note that our contrib guide encourages a fairly specific commit structure. I've written up more on it at https://epage.github.io/dev/pr-style/#c-test