Allow variable references in depends_on

[dbanck]

This PR allows variable references in depends_on aligning it with Terraform.

UX

Before

After

---

• Resolves Variable references are not recognised in depends_on vscode-terraform#1598

[radeksimko]

I believe that local references are also allowed but we can address that in a separate PR.

locals {
  foo = true
}

resource "random_pet" "name" {
  depends_on = [local.foo]
}

^ that is considered valid by plan/apply/validate but we also raise it as an error:

Whether it's a good practice and good idea is another question, but it's at least ignored/allowed by Core.