Dependency on Magento was sneaked in as part of the Computed Resolver work #90
Description
This package has no declared composer dependencies on Magento, yet in 6d25b49 a dependency is hardcoded.
This means that it cannot be considered anymore as a standalone implementation of upward in PHP.
Following Dependency Inversion principle we should create an interface which \Magento\UpwardConnector\Model\ComputedPool implements, or make \Magento\UpwardConnector\Model\ComputedPool part of the magento/upward-php together with \Magento\UpwardConnector\Api\ComputedInterface.