root_package returns None if no_deps is passed

[zombiepigdragon]

Minimal Reproducible Example:

src/main.rs

use cargo_metadata::MetadataCommand;

fn main() {
    MetadataCommand::new()
        .exec()
        .unwrap()
        .root_package()
        .expect("root package exists");
    MetadataCommand::new()
        .no_deps()
        .exec()
        .unwrap()
        .root_package()
        .expect("root package exists w/ no_deps called");
}

Cargo.toml

[package]
name = "root_package_mre"
version = "0.1.0"
edition = "2021"

# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html

[dependencies]
cargo_metadata = "0.15.0"

The above crate panics at the second expect when run on itself. This is unexpected because there is a clear root crate and the choice to include/exclude dependencies doesn't seem like it should affect its detection.

The documentation for MetadataCommand states "Output information only about the root package", but this is incorrect in the presence of workspaces: I ran into this while loading metadata for ripgrep, which at this time has a root package and a workspace. This crate identifies all of the workspace's packages with no_deps, running counter to the documentation, though this is almost certainly the correct behavior.

This occurs because no_deps leaves Metadata.resolve as None, and root package detection assumes the resolver will identify the package. I have not yet looked at the raw output of cargo metadata and so do not know if there is an alternative way to find the root package.

If this behavior is can't/WONTFIX, a documentation update to Metadata::root_package noting this case would be useful.