Unnecessary validation of unmodified resources in same dataset

# Overview

When a resource is modified, all resources in the dataset are validated, even though most of them are unmodified. On datasets with many resources, this can result in a substantial performance problem, especially if the resources are large.

It appears that the `after_update` function assumes that the presence of "resources" in the data dictionary means that the whole package is being updated at once. However, this is not necessarily the case. If a resource is updated via eg the `resource_update` API, then this code path will still trigger.

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