terraform_required_providers: emit error when only source is specified

[bendrucker]

In Terraform 0.13+, all providers not in the hashicorp namespace that are loaded from the Terraform Registry require a source. This means a user might likely to have the following working config:

terraform {
  required_providers {
    datadog = {
      source = "datadog/datadog"
    }
  }
}

The intent of this rule is to require modules to specify their provider version requirements. Currently, the above is accepted since there's an entry in required_providers. Checking for a non-nil version constraint ensures that the version has been included in the provider source.