Stack Dependency Mock Outputs

Stack dependencies honor mock_outputs with --dependency-fetch-output-from-state

A dependency block that reads outputs from a stack (its config_path points at a terragrunt.stack.hcl directory) used to fail when a unit in that stack had no state yet, even when the dependency declared mock_outputs. This blocked commands like plan and validate against a stack that hadn't been applied.

Such a dependency now falls back to mock_outputs for the units that have no state yet. In a partially applied stack, applied units resolve to their real outputs while the rest use their mocks.

Mocks for a stack dependency are keyed by unit name, so mock_outputs has to be a map or object. Declaring it as any other type now reports that directly, instead of leaving the units it can't cover out of the stack outputs.