A resource whose definition was dropped leaves no role that can be saved

Problem/Motivation

This module removes a resource from the plugin definitions when the reason for it is gone: the optional module it declares is not installed, the convention role it needs is empty, the bundle it serves was deleted. A site that had enabled that resource keeps its rest_resource_config entity, and the router was already made safe for that: the configuration is loaded disabled, the cache rebuild goes through and the endpoint answers 404.

The permission list was not. RestPermissions::permissions() asks every resource configuration for its plugin - enabled or not - with no guard, so a single configuration naming a missing plugin throws PluginNotFoundException out of user.permissions. Measured on a site: the permissions page cannot be built, and saving any role at all fails, because a role calculates its dependencies from the permission list. Nothing on the page says which configuration is to blame.

Proposed resolution

• Make the permission list survive a configuration whose plugin is gone, in one place, for every case this module drops a definition in - the same promise the router already has.
• Say which configuration names a missing plugin where a site owner looks: the status report, with the command that removes it.
• Coverage: a configuration naming a missing plugin, and the permission list builds, a role can be saved and the permissions page renders.

Remaining tasks

Everything.