The step generator applies `ignoreChanges` pre-processing for all
resources by copying old input values to the new goal for any properties
mentioned in the `ignoreChanges` list. However, this pre-processing
depends on the existence of prior inputs, which by definition does not
exist for a resource being imported prior to the issuance of the
`ImportStep`. These changes add this processing to the implementation of
`ImportStep`, using the inputs read from the provider as the prior
inputs.
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