static File applicationFile(File parent, Path path) {
        File file = new File(parent, path.getRelative());
        if ( ! file.getAbsolutePath().startsWith(parent.getAbsolutePath()))
            throw new IllegalArgumentException(file + " is not a child of " + parent);

        return file;
    }
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