How can I install composer packages with drush make? How does one include composer package in a drush make file?

The more general answer, the better. There's already a composer drush module allowing drush dl to handle composer packages, but that requires a shell command for every dependency.

Given our future use of Symfony, composer and other Symfony bundles is likely to become more important in the future.
You should use yoursite.make to download the Drupal module and composer.json for composer.

There is the Composer module that integrates Composer with Drush.

I hope that soon .make is going to disappear, replaced by composer.json; for that to come true, some issues need to be resolved, such as version number.

Composer packages are stored in the git repository; you can't use a composer package, but you can download the code. For example, for https://packagist.org/packages/symfony/symfony you can use the following.

libraries[symfony][download][type] = "git"
libraries[symfony][download][url] = " https://github.com/symfony/symfony.git"
libraries[symfony][destination] = "lib/symfony/"


Composer can build stuff recursively; of course, this will not work here.

See: http://cgit.drupalcode.org/drush/plain/docs/make.txt

Their is also https://github.com/composer/installers than can do the invert (install drupal module with composer).