During one of many discussions with stronk7 about MDL-49329, we realized that the \core\update\validator class could precheck for the dependencies of the plugin and raise a warning if the installed plugin depends on another plugin that is not available in the plugins directory.

This is doable. We would need to add the support for parsing the $plugin->dependencies declaration and check for them (somehow) during the validation.

It does not seem to be a crucial issue as the admin has a chance to cancel the installation/upgrade if they realize the requirements cannot be fulfilled. So leaving this open to see if there is a demand for such pre-checking.