terraform_required_providers: use required_providers entry as range where possible

[bendrucker]

When a required provider has a source but not a version, the rule still emits the error on the provider call:

tflint-ruleset-terraform/rules/terraform_required_providers_test.go

Line 127 in df0f6ca

provider "template" {}

In this case, the entry in required_providers would be a more suitable range. This would enable the following:

terraform {
  required_providers {
    # tflint-ignore: required_providers
    foo = {
      source = "host/foo"
    }
  }
}