code: function Package(opts) {
    this.path = opts.path;
    this.files = opts.files;
    this.metadataTypeXmlNames = opts.metadataTypeXmlNames;
    this.subscription = opts.subscription;
    this.project = opts.project;
  }
docs: can take array of metadata as constructor argument, turns into object representation { "ApexClass" : [ "thisclass", "thatclass" ], "ApexPage" : "*" }