Given these realities:
* A user //should// be able to use Composer to install extensions if they want to, but nobody should be forced to use it
* Most extensions do not have a valid `composer.json` file, nor want to add one.
* Most extensions are not on [[ https://packagist.org/ | Packagist.org ]] (and you could certainly argue that they shouldn't be).
* If an extension is not on Packagist, a user must manually specify the [[ https://getcomposer.org/doc/05-repositories.md#vcs | VCS repository ]] in their `composer.json`
A Wikimedia [[ https://getcomposer.org/doc/05-repositories.md#hosting-your-own | Composer Repository ]] should be created which resolves all of the issues above.
This is similar to what Drupal.org has done for the Drupal extensions:
https://www.drupal.org/node/2718229#drupal-packagist