Why not do it like this and collect them using nix?

use rayon::iter::ParallelIterator;

#[allow(dead_code)]
#[derive(serde::Serialize)]
struct CrateJson {
    name: String,
    version: String,
    checksum: [u8; 32],
    deps: Vec<String>,
}

fn main() {
    let index = crates_index::GitIndex::with_path("../index", crates_index::git::URL).unwrap();

    println!("Processing crates...");
    index
        .crates_parallel()
        .filter_map(|c| c.ok())
        .for_each(|c| {
            println!("Processing crate: {}", c.name());

            let name = c.name();
            let versions = c.versions();

            let mut major_versions: std::collections::HashMap<u64, Vec<&crates_index::Version>> =
                std::collections::HashMap::new();

            for v in versions {
                if let Ok(semver) = semver::Version::parse(v.version()) {
                    if semver.major == 0 {
                        let key = semver.minor + 1000000;
                        major_versions.entry(key).or_default().push(v);
                    } else {
                        major_versions.entry(semver.major).or_default().push(v);
                    }
                }
            }

            for (_, mut group) in major_versions {
                group.sort_by(|a, b| {
                    let va = semver::Version::parse(a.version()).unwrap();
                    let vb = semver::Version::parse(b.version()).unwrap();
                    vb.cmp(&va)
                });

                if let Some(latest) = group.first() {
                    let semver = semver::Version::parse(latest.version()).unwrap();
                    let major_ver = if semver.major == 0 { 0 } else { semver.major };

                    let prefix = if name.len() >= 2 { &name[..2] } else { name };

                    let dir_path = format!("rust-crates/by-name/{}/{}", prefix, name);
                    std::fs::create_dir_all(&dir_path).unwrap();

                    let file_path = format!("{}/v{}.json", dir_path, major_ver);

                    let crate_json = CrateJson {
                        name: name.to_string(),
                        version: latest.version().to_string(),
                        checksum: *latest.checksum(),
                        deps: latest
                            .dependencies()
                            .iter()
                            .map(|d| d.name().to_string())
                            .collect(),
                    };

                    let json = serde_json::to_string(&crate_json).unwrap();
                    std::fs::write(&file_path, json).unwrap();
                }
            }
        });
}