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.
Rather than providing an already-resolved map of plugins to core, we now
provide a "provider resolver" which knows how to resolve a set of provider
dependencies, to be determined later, and produce that map.
This requires the context to be instantiated in a different way, so this
very noisy diff is a mostly-mechanical update of all of the existing
places where contexts get created for testing, using some adapted versions
of the pre-existing utilities for passing in mock providers.
Fixes#11212
The import graph builder was missing the transform to setup links to
parent providers, so provider inheritance didn't work properly. This
adds that.
This also removes the `PruneProviderTransform` since that has no value
in this graph since we'll never add an unused provider.
