Deprecate Include Without Label

[yhakbar]

For backwards compatibility, we support the ability to define include blocks without labels.

This results in a double parse of HCL files, as the HCL parser requires the exact number of labels to be known for every include block:
https://github.com/gruntwork-io/terragrunt/blob/a02986/config/hclparse/file.go#L56

Similar to the deprecation of inputs from dependencies, we can provide a performance improvement for users that aren't using this legacy syntax by introducing a strict control that prevents usage of include blocks without labels, and skipping the secondary parse.

Given that Terragrunt has supported this for a long time, we shouldn't look to remove support for includes without labels until well after 1.0.