Add Support for Terraform Modules in Terraform Parser

[abhisek]

The current terraform parser supports parsing .terraform.lock.hcl to extract Terraform Providers. We need to extend it to be able to collect Terraform modules as well.

See following for more details: https://github.com/safedep/vet/blob/main/pkg/parser/terraform.go

Once this support is added, vet should be able to identify a Terraform module using its internal package model. Refer models/models.go.

Handling module is not straight forward because

At present, the dependency lock file tracks only provider dependencies.

https://developer.hashicorp.com/terraform/language/files/dependency-lock

So we have a few options:

• Leverage .terraform/modules/modules.json which seems to be created by terraform init if a TF configuration uses modules
• Parser .tf files and look for remote modules. We are already parsing HCL, so parsing .tf for modules should not be a problem

Example module definition:

module "eks" {
  source  = "terraform-aws-modules/eks/aws"
  version = "20.28.0"
}

We are interested only in remote modules i.e. those that do not start with . or .. Read module about module sources at: https://developer.hashicorp.com/terraform/language/modules/sources