Fatal error caused by install_check_class_requirements

Problem/Motivation

Found while working on #3404246: [META] Fix strict type errors detected by phpstan.

This isn't doing what it looks like. In install_check_class_requirements():

if (class_exists($class_name) && class_implements($class_name, InstallRequirementsInterface::class)) { }

The second param of class_implements is a boolean to opt in or out of autoloading, and the return value is an array of interfaces the class implements. So as long as the class implements one or more interfaces, this condition is satisfied, regardless of if its the right interface. This is why strict types is so useful and why I'm pushing for the phpstan check in the meta.

Steps to reproduce

1. Install standard profile
2. Edit \Drupal\media\Install\Requirements\MediaRequirements to implement some other interface instead
3. Remove MediaRequirements::getRequirements
4. Install the media module and see the fatal error

Proposed resolution

Replace call to class_implements with is_subclass_of

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet