If you have a module which contains a composer.json, you want to install the dependencies during the build process.

In platform.sh you have something called hooks, which are executed during the deployment. To execute a ‘drupal composer-update’ during deployment, you have to add the following to your .platform.app.yaml:

 build: |
   cd public
   php modules/contrib/composer_manager/scripts/init.sh
   composer drupal-update

Now all the dependencies from all the composer.json files you have, will be downloaded after platform.sh has built your makefile.