We're going to use config to determine provider dependencies, so we need
to always provide a config when instantiating a context or we'll end up
loading no providers at all.
We previously had a test for running "terraform import -config=''" to
disable the config entirely, but this test is now removed because it makes
no sense. The actual functionality its testing still remains for now,
but it will be removed in a subsequent commit when we start requiring that
a resource to be imported must already exist in configuration.
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