Pony currently handles installation of first-gen packages.

You can find first-gen packages in the file data/groups/packages.json. This file is a simple JSON which maps a name of a package group to a list of package definitions. Each package definition has to include at least SubscriberPackageName and SubscriberPackageVersionId. If you have some org with all packages installed, use sfdx pony:package:group:export to create this file. This command automatically removes standard packages, please open an issue if some standard package is not removed.

{
    "default": [
        {
            "SubscriberPackageName": "Package Name",
            "SubscriberPackageVersionId": "04t..."
        }       
    ]
}