How do I package a distribution on Drupal.org?
On Drupal.org, there isn't yet support for a distribution composer.json file, as reported on Support for distributions.
The package system used from Drupal.org, to be able to build the package files for a distribution, requires just two files: drupal-org.make and drupal-org-core.make. The latter is only necessary when using a Drupal release that is not official.
If your system doesn't allow you to create a .make file as required from Drupal.org, you can manually create it with any text editor, as long as you use the correct file format (see Example drupal.org make file), and you follow what reported in Drupal.org distribution packaging requirements. In particular, see the notes reported at the beginning.

External libraries are validated against a whitelist of GPL-compatible libraries since all code distributed from Drupal.org must be GPL compatible
Git clones from Drupal.org sandboxes (as opposed to full projects) are not supported
Patches hosted on servers other than Drupal.org are not supported
Modules and themes hosted on servers other than Drupal.org are not supported
If you need to specify anything other than an official release of the Drupal core project itself, that needs to go into a separate drupal-org-core.make file
If you request a specific Git revision you must also define the branch that revision came from

In future, some of those restrictions could be lift. There are feature requests / bugs opened in the packages.drupal.org issue queue.