Stack Dependencies Remove Nested Refs

stack-dependencies: simplified unit.* / stack.* ref shape

The stack-dependencies experiment no longer resolves stack.<name>.<unit_name>.path or stack.<name>.<nested_stack>.path. Only the top-level stack.<name>.path and unit.<name>.path forms remain. stack.<name>.name and unit.<name>.name are gone too; both only ever echoed the label that the reference already had to spell out.

Nested references required parsing every nested catalog up front and conflicted with the reserved name and path attributes on each ref: a nested unit named name or path could not be addressed.

To depend on a generated unit inside a stack, compute the path as ${stack.<name>.path}/<unit-relative-path> directly. The layout under a stack's generated directory follows no_dot_terragrunt_stack on the parent stack and on each unit, so hand-computed paths must mirror that resolution.