support terraform v0.12.N

[ryandeivert]

Background

Terraform 0.12.X requires explicit relative paths (using a ./ prefix). See: poseidon/typhoon#457.

This breaks things, and we should update to support the newest release of terraform.

[ryandeivert]

I was looking to see why this check was not working, and verified locally that module resolution happens before the version check.... wtf.

Test Steps

• Use tfenv to set terraform version to 0.12.2
• Added a file to terraform/ directory named main.tf with contents:

  
  terraform {
  required_version = ">= 0.12.0"
  }

module "test" { source = "modules/foo_bar_test_modules" // implicitly relative path }

* Ran `terraform init` and got error
* Changed implicitly relative path to explicit via: `./modules/foo_bar_test_modules`
* Ran again and no error

[ryandeivert]

complete with #1035