Problem/Motivation
In Drupal 8, contributed modules may require Composer dependencies, but Drupal core itself will not require site builders and developers to use Composer to assemble a site's code base. This means that contributed modules currently have to ensure their Composer dependencies are installed themselves, which is a task too complex and important to reinvent in every project.
Proposed resolution
Use composer/semver and Composer's installed.json to check if the packages defined in the custom/contrib module are installed before allowing module installation.
Remaining tasks
None.
User interface changes
One additional Composer item will appear in the status report, indicating whether all necessary Composer dependencies have been installed or not. This is the same requirement that can prevent modules from being installed.
API changes
None.