Highlight unused provider which has an alias

[wtw-joecooper]

Similar to terraform_unused_declarations but it highlights when a provider with an alias hasn't been used.

The potential issue is that I've primarily used a AzureRM and I'm not aware of how other providers are used and if this might cause issues.

Example:

Fail:

provider "azurerm" {
  features {}
  alias           = "test_123"
  subscription_id = ""
}

Pass:

provider "azurerm" {
  features {}
  alias           = "test_123"
  subscription_id = ""
}

resource "azurerm_resource_group" "example" {
  name     = "example-resources"
  location = "West Europe"
  provider = azurerm.test_123
}

[bendrucker]

Good catch, added to unused_declarations in #304. Since aliases must be explicitly referenced, we can detect those provider blocks when unused.