feat: add completions for install --path (#15266)

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### 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).

https://github.com/user-attachments/assets/281706c0-7956-4551-a22b-125dadccd12e

### Additional information

There's a [discussion in
Zulip](https://rust-lang.zulipchat.com/#narrow/channel/246057-t-cargo/topic/PR15266.3A.20completions.20for.20install.20--path)
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.

Author: epage

src/bin/cargo/commands/install.rs

@@ -65,7 +65,11 @@ pub fn cli() -> Command {
         .arg(
             opt("path", "Filesystem path to local crate to install from")
                 .value_name("PATH")
-                .conflicts_with_all(&["git", "index", "registry"]),
+                .conflicts_with_all(&["git", "index", "registry"])
+                .add(clap_complete::engine::ArgValueCompleter::new(
+                    clap_complete::engine::PathCompleter::any()
+                        .filter(|path| path.join("Cargo.toml").exists()),
+                )),
         )
         .arg(opt("root", "Directory to install packages into").value_name("DIR"))
         .arg(flag("force", "Force overwriting existing crates or binaries").short('f'))