infracost: HCL parsing does not work with modules that have a `source` in a private Terraform registry

HCL parsing does not work if the Terraform code contains a module with a source in a private Terraform registry. Private git modules work ok.

Example this code:

provider "aws" {
  region                      = "us-east-1"
  skip_credentials_validation = true
  skip_requesting_account_id  = true
  access_key                  = "mock_access_key"
  secret_key                  = "mock_secret_key"
}

module "ec2_cluster" {
  source  = "app.terraform.io/infracost/ec2-instance/aws"
  version = "~> 2.0"

  name           = "my-cluster"
  instance_count = 5

  ami                    = "ami-ebd02392"
  instance_type          = "t2.micro"
  key_name               = "user1"
  monitoring             = true
  vpc_security_group_ids = ["sg-12345678"]
  subnet_id              = "subnet-eddcdzz4"
}

Shows this error:

Error: Error loading Terraform modules: error downloading 'file:///examples/terraform/.infracost/terraform_modules/ec2_cluster/app.terraform.io/infracost/ec2-instance/aws': source path error: stat /examples/terraform/.infracost/terraform_modules/ec2_cluster/app.terraform.io/infracost/ec2-instance/aws: no such file or directory

There’s two issues which are causing this:

1. Our code assumes that the module API is always at <host>/v1/modules but this can be different for different registries. We should use the terraform-svchost package to discover the correct service URLs
2. Our code does not pass through any credentials when pulling remote registry modules.

I’ve created a work in progress branch here.