[bug] module_header: Export list is sorted but individual elements on that list are not sorted

Example in https://github.com/input-output-hk/ouroboros-network/blob/master/ouroboros-consensus-cardano/src/Ouroboros/Consensus/Cardano/Block.hs#L19

# Example

## Input
```
module Ouroboros.Consensus.Cardano.Block (
HardForkBlock (
BlockByron
, BlockShelley
, BlockAllegra
, BlockMary
)
, CardanoBlock) where
```

## Expected

```
module Ouroboros.Consensus.Cardano.Block (
CardanoBlock
, HardForkBlock (BlockAllegra, BlockByron, BlockMary, BlockShelley)
) where
```

## Actual

```
module Ouroboros.Consensus.Cardano.Block (
CardanoBlock
, HardForkBlock (BlockByron, BlockShelley, BlockAllegra, BlockMary)
) where
```

Start with the module_header formatting logic and reproduce the issue using ouroboros-consensus-cardano/src/Ouroboros/Consensus/Cardano/Block.hs at line 19. Done means nested elements in an export list are sorted alphabetically, matching the expected output shown here.

Written by the indexing model from the issue text.