Ensure plugins are reloaded after an update

Problem/Motivation

We want to ensure that plugins are reloaded after an update.

To be clear, this means that we expect plugin definitions to change when the update is complete, but plugin implementations (i.e., the class itself) will be the same. That's weird, I know, but it's a limitation of the PHP runtime.

Proposed resolution

Add a block plugin to updated_module and change both its definition and implementation in the updated version of the module. Post-update, the definition should be changed, but the implementation should not.

Remaining tasks

Do it.