How to use the @salesforce/core.Aliases.fetch function in @salesforce/core

}
    if (args.instanceUrl || args.i) {
        config.auth.instanceUrl = args.instanceUrl || args.i;
    }
    if (args.outputFile || args.o) {
        config.outPath = args.outputFile || args.o;
    }

    if (args.sObjects || args.s) {
        config.sObjects = (args.sObjects || args.s).split(',');
    }

    if (config.auth.accessToken === undefined) {
        // if just username is set, load from sfdx
        if (config.auth.username !== undefined && config.auth.password === undefined) {
            let username = await Aliases.fetch(config.auth.username);
            if (username) {
                config.auth.username = username;
            }
            console.log(config.auth.username);
            let connection: Connection = await Connection.create({
                authInfo: await AuthInfo.create({ username: config.auth.username }
                )
            });
            let org = await Org.create({ connection });
            await org.refreshAuth();
            connection = org.getConnection();

            config.auth.accessToken = connection.accessToken;
            config.auth.instanceUrl = connection.instanceUrl;
        } else if (config.auth.username !== undefined && config.auth.password !== undefined) {

public static async getUsername(
    usernameOrAlias: string
  ): Promise {
    const username = await Aliases.fetch(usernameOrAlias);
    return Promise.resolve(username);
  }