Support Terragrunt dependencies between projects

[ZIJ]

Raised by Bastien: regarding terragrunt, I think you don't have the concept of dependencies yet. Right now I'm parsing hcl files to generate digger.yml with the correct include_patterns . Is there a way to make sure plan/apply are made in the correct order ?

Igor: indeed no dependencies support yet (...) there are similar concepts in Spacelift and the likes (...)
it looks like there could be somewhat different use cases for dependencies:

1. the include_patterns workaround solving for project A apply being triggered whenever changes are made to an outside directory, like modules
2. "soft" dependencies between projects - smth like this: whenever apply for BOTH project A and B are triggered, A always runs before B. Not preventing isolated runs of either A or B though.
3. "hard" dependencies between projects - whenever apply B is triggered, apply A must run first (even if no changes to A were made)

Bastien: 2 should be enough to support terragrunt dependencies

[ZIJ]

Additional input from Bastien:

(Bastien): Even if it's "usable" it's something that we need if we want to use it in production. Terragrunt will fail if a dependency needs to be applied. In this case we need to (manually) make sure project A is applied before B. So for if we touch a project (e.g VPC) which many projects depends on, it becomes difficult to manage, it's way easier to just run terragrunt run-all apply rather than digger apply -p for every project (and I'm not even mentioning dependencies of dependencies :rolling_on_the_floor_laughing: )

Q (Igor): with dependency feature in place (we're thinking a standard dependency graph flattening logic), would you want to still run the terragrunt binary for each project, or terraform? or both (differently for different projects)?

A (Bastien): terragrunt, still need it to load common config/etc.

[ZIJ]

Related to #924 and #791