Replace terragrunt-atlantis-config with <something>

[ZIJ]

We currently use a slightly modified version of the terragrunt-atlantis-config library for parsing of Terragrunt projects. This has several drawbacks:

• It's slow. One user reported that parsing 1400 terragrunt projects (using Atlantis) takes more than 5 minutes
• It doesn't respect dependencies. Same user reported that they have to raise multiple PRs in specific order to avoid errors. Atlantis solution: execution order groups

We probably need to move away from terragrunt-atlantis-config. But what to replace it with? A few options come to mind:

• Treat each Terragrunt project as a "black box" - let Terragrunt manage it. But that would limit it to a single job; no concurrency, no scale for Terragrunt then - which kind of defies the purpose of solving the issue, because it's not going to be slow at small scale anyway and cross-project dependencies are unlikely to be a pressing problem
• Contribute to Terragrunt to expose some kind of a "project status" api so that external tools could get the status of projects without actually running terragrunt commands
• Use the existing graph-dependencies Terragrunt command
• Build our own DAG parser using this lib or smth else

Related to:

• Support terragrunt run-all #962
• generate_projects does not respect Terragrunt dependency order #924
• Support Terragrunt dependencies between projects #402
• Dependencies #791
• Supporting generation of projects for terragrunt workspaces #424

Reported by the user C.D.

[norman-zon]

In any case please keep the functionality of atlantis_extra_dependencies. We rely heavily on them.

[ben-of-codecraft]

@ZIJ We have run into the dependency one. I noticed in the code it does look like it is attempting to use ExecutionOrderGroups now. Is part of this been implemented?