private static void WriteSpecFile(NugetSpec spec, DirectoryInfo packageDir)
        {
            var specFilename = spec.Id + "." + spec.Version + ".nuspec";
            var specFilePath = Path.Combine(packageDir.FullName, specFilename);

            using (var writer = new XmlTextWriter(specFilePath, Encoding.UTF8) {Formatting = Formatting.Indented})
            {
                spec.GetXml().WriteTo(writer);
            }
        }