Stack Output Nested Key Order

Fixed nested key order in terragrunt stack output

When a unit lived inside more than one nested stack, terragrunt stack output rendered its key with the stack names reversed, so a unit inside root_stack_3 > stack_v3 > stack_v2 appeared under stack_v2.stack_v3.root_stack_3 instead of root_stack_3.stack_v3.stack_v2. Deeply nested units also leaked to the top level of the output.

The output now joins stack names from outermost to innermost, matching the declared hierarchy in both the HCL and JSON formats.

Thanks to @anuragrao04 for contributing this fix!