When calculating deletes, we will only issue a single delete step for a
particular URN. This is incorrect in the presence of pending deletes
that share URNs with a live resource if the pending deletes follow the
live resource in the checkpoint: instead of issuing a delete for
every resource with a particular URN, we will only issue deletes for
the pending deletes.
Before first-class providers, this was mostly benigin: any remaining
resources could be deleted by re-running the destroy. With the
first-class provider changes, however, the provider for the undeleted
resources will be deleted, leaving the checkpoint in an invalid state.
These changes fix this issue by allowing the step generator to issue
multiple deletes for a single URN and add a test for this scenario.
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