Upgrade/Install: Introduce Plugin Dependencies.
Introduces a new "Requires Plugins" plugin header so that plugin developers can list the slugs of the plugins theirs depends on.
This will inform users of the requirements, and provide links to the WordPress.org Plugins Repository that they can click to install and activate the dependencies first.
Plugins whose requirements are not met cannot be installed or activated, and they will be deactivated automatically if their requirements become unmet.
Plugins that others rely on cannot be deactivated or deleted until their dependent plugins are deactivated or deleted.
In memory of Alex Mills and Alex King.
WordPress Remembers.
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Fixes #22316.