fix(autodetect): only use directories that contain var files when associating children

[hugorut]

Resolves issues with autodetect where we fail to properly associate child Terraform var files if the root directory also has children that contain root directories. Take the following example:

.
└── terraform
    ├── config
    │   ├── dev
    │   │   └── terraform.tfvars
    │   └── prod
    │       └── terraform.tfvars
    ├── main.tf
    └── foo
        ├── main.tf
        └── config
            ├── dev
            │   └── terraform.tfvars
            └── prod
                └── terraform.tfvars

In this case, prior to this change, when we called AssociateChildVarFiles on the terraform directory we stopped traversing at the depth of the foo directory. This meant that we were not able to associate config/dev/terraform.tfvars to the project.

I've now made a fix to add a method ChildTerraformVarFiles which traverses the tree only returning directories that contain only tfvar files. This is safe to use in the AssociateChildVarFiles context as this is only concerned with traversing to find child var files.