feat: allow using str in HashMaps with a PackageName key

[baszalmstra]

Adds a Borrow<str> implementation for PackageName.

This allows indexing HashMaps with a PackageName as key with a str. e.g.:

fn get_package_version(map: &HashMap<PackageName, Version>) -> Option<&Version> {
    maps.get("normalized_package_name")
}

just works.

Previously you would have to write:

fn get_package_version(map: &HashMap<PackageName, Version>) -> Option<&Version> {
    maps.get(PackageName::from_str("normalized_package_name").unwrap())
}

Similar to prefix-dev/rip#89