fix: set source in required_providers

[bendrucker]

What does this PR do?

Replaces the legacy required_providers declaration with {source, version}.

Motivation

Fixes warnings about implicit provider selection when passing in a provider instance to the module. Closes #31.

Limitations

Since Terraform continues to support this syntax for backward compatibility with 0.12, we need to enforce this separately. terraform-linters/tflint-ruleset-terraform#64 will allow us to detect and reject this syntax in the future.

Testing

I tried to write a test module but was unable to reproduce the originally reported warning on terraform validate:

module "m" {
  source = "../"

  observe_token = "id:secret"
  observe_customer = "12345"

  providers = {
    aws = aws.aliased
   }
}

provider "aws" {
  alias = "aliased"
}

Even if there were no reported issue/warning, we should still be setting source anyway.