feat: add completions for install --path

[nooma-42]

What does this PR try to resolve?

Related to #14520

This PR introduces auto-completion for the cargo install --path option. When a user types cargo install --path and presses the TAB key, the system will automatically detect directories in the current path and suggestion prioritizes directory containing a Cargo.toml file.

How should we test and review this PR?

To verify this feature, follow these steps:

1. In the terminal, type cargo install --path
2. Press the TAB key.
3. You should see suggestions only for directories in the current path that contain a Cargo.toml file being prioritized, such as ./src/ (if ./src/Cargo.toml exists).

2025-03-08.1.08.06.mov

Additional information

There's a discussion in Zulip and here're some relevant information from it

1. directories are always shown so that a completer doesn't have to recurse through a directory tree to determine what directories may be relevant. The difference marking a directory as accepted does it sorts it first.
2. A user / config setting might override a sort order over native completion. Try disable it manually if native completion is not there.

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[nooma-42]

@epage @weihanglo
Thanks for your review :) This is my first time contributing to Cargo, so I'm not very familiar with it. Please bear with me if this is a dumb question.
I wanted to ask if cargo native-completion can print debug messages. I followed the instructions here and added some environment variables, but nothing got logged to a file or printed in the terminal.

https://doc.crates.io/contrib/implementation/debugging.html

[weihanglo]

I wanted to ask if cargo native-completion can print debug messages. I followed the instructions here and added some environment variables, but nothing got logged to a file or printed in the terminal.

If you're adding tracing log like tracing::info!, try exporting CARGO_LOG environment variable, so your subsequent cargo call can see the variable. For example,

# zsh
export CARGO_LOG=info
cargo install --path <tab>

If you feel logging too hassle to use, use dbg! or just panic! so you'll see it without setting any env var.

[epage]

Anything written to stdout will be read by the completion shim and interpreted as completions to report to the user
https://github.com/clap-rs/clap/blob/fdbbf66e77c83688f52b7a206d64102582af40d3/clap_complete/src/env/shells.rs#L40-L46

Unsure what else a shell's completion system may do on top of what we're doing.

You can also directly invoke the completer so nothing gets captured. You can run CARGO_COMPLETE=zsh cargo +nightly, see how cargo is invoked and do similar.

[weihanglo]

Mind fixing up the commits for how you prefer them to get merged?

[epage]

Feel free to squash your commits for how you would like them to be merged

[epage]

Thanks!