fix: prevent crash for legacy provider lookups

[radeksimko]

Fixes #479

This accounts for a case where users don't have explicit provider requirement in their configuration, but use Terraform 0.13+ (which outputs provider schemas with new addresses).

Generally ProviderSchema() is not meant to ever return nil, nil, which other logic in terraform-schema relies on:

https://github.com/hashicorp/terraform-schema/blob/ad0461b4382716039f6d35e8fd9bd8599458285c/schema/schema_merge.go#L70-L77

I also added some extra logging in "hot paths" which users are likely to hit and where knowing more may aid debugging in the future.

Legacy addresses will unfortunately not be eliminated in the near future, despite how we refer to them (legacy) as there's currently no plan for making explicit source argument required in Terraform v1.

We can guide people to help eliminate it #480 and convert legacy addresses ourselves #458

Regardless though we need to be able to deal with such legacy addresses in the meantime.

[ghost]

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