core: use correct fallback provider address for abstract resource instance
core: use correct fallback provider address for abstract resource instance

Previously we were attempting to construct a default manually here, and
actually getting it wrong by using the resource type name as a whole
rather than the expected inferrence by prefix.

Now we use the method provided in the addrs package for this purpose,
which implements the standard behavior of shaving off the first
underscore-separated word from the resource type name.

hashicorp/terraform-plugin-sdk,hashicorp/terraform-plugin-sdk