A resource can be imported by setting the `import` property in the
resource options bag when instantiating a resource. In order to
successfully import a resource, its desired configuration (i.e. its
inputs) must not differ from its actual configuration (i.e. its state)
as calculated by the resource's provider.
There are a few interesting state transitions hiding here when importing
a resource:
1. No prior resource exists in the checkpoint file. In this case, the
resource is simply imported.
2. An external resource exists in the checkpoint file. In this case, the
resource is imported and the old external state is discarded.
3. A non-external resource exists in the checkpoint file and its ID is
different from the ID to import. In this case, the new resource is
imported and the old resource is deleted.
4. A non-external resource exists in the checkpoint file, but the ID is
the same as the ID to import. In this case, the import ID is ignored
and the resource is treated as it would be in all cases except for
changes that would replace the resource. In that case, the step
generator issues an error that indicates that the import ID should be
removed: were we to move forward with the replace, the new state of
the stack would fall under case (3), which is almost certainly not
what the user intends.
Fixes#1662.
e1a52693dc