Feature request: Index workspace

[gilescope]

Would be cool to point this at a large workspace and have it index all the crates it finds there.

I did something similar for undepend. Something like this should do the trick:

use cargo_metadata::{Metadata, MetadataCommand};

 let metadata = MetadataCommand::new()
        .manifest_path(repo_dir.join("Cargo.toml"))
        .exec()
        .unwrap();

 for package in metadata.workspace_members.iter() {
}