Fix attribute lookup for dependent schemas

[radeksimko]

This fixes a conversion function which was previously not covered by tests.

This bug in combination with another bug hashicorp/terraform-schema#28 caused the dependent schemas of resource or data block to be looked up basically freely without the provider attribute being considered.

In other words most of the time it looked like everything is working fine, but there were some configurations being misinterpreted, e.g.

terraform {
  required_providers {
    rand = {
      source  = "hashicorp/random"
      version = "3.0.0"
    }
  }
}

resource "random_string" "name" {
  provider = rand
}

It is true that providers of hashicorp (default) namespace do not require explicit required_providers entry, however if they do have one, it should be respected.

Here due to the combination of the two bugs we would look up hashicorp/random schema for random_string resource block, but that provider really isn't supposed to be available under that name. It requires explicit association.