public FilesApplicationPackage build() {
            return new FilesApplicationPackage(appDir, preprocessedDir.orElse(applicationFile(appDir, preprocessed)),
                                               metaData.orElse(readMetaData(appDir)), includeSourceFiles);
        }
Builder for {@link com.yahoo.config.model.application.provider.FilesApplicationPackage}. Use
this to create instances in a flexible manner.
config-application-package/src/main/java/com/yahoo/config/model/application/provider/FilesApplicationPackage.java