Immediately call `ref.onCancel` after the initialization of a provider if it isn't currently listened

[rrousselGit]

Currently, calling ref.read on a provider that isn't listened yet (no ref.watch/listen) does not trigger ref.onCancel. Yet at the same time, that provider is considered to be paused, as it isn't currently in use.

This issue is about fixing the inconsistency, and invoking ref.onCancel listeners if the provider is initialized but not listened