Preserve split order in DataFilesDict
After investigation, I have found that this copy forces the splits to be sorted alphabetically: https://github.com/huggingface/datasets/blob/029227a116c14720afca71b9b22e78eb2a1c09a6/src/datasets/builder.py#L556

This PR removes the alphabetically sort of `DataFilesDict` keys.
- Note that for a `dict`, the order of keys is relevant when hashing:
  ```python
  hash1 = Hasher.hash({'train': 'train.csv', 'test': 'test.csv'})
  hash2 = Hasher.hash({'test': 'test.csv', 'train': 'train.csv'})
  assert hash1 != hash2
  ```
- The `DataFilesDict` is a subclass of `dict`, thus the order should be relevant as well
  ```python
  hash1 = Hasher.hash(DataFilesDict({'train': 'train.csv', 'test': 'test.csv'}))
  hash2 = Hasher.hash(DataFilesDict({'test': 'test.csv', 'train': 'train.csv'}))
  assert hash1 != hash2
  ```

Fix #6196.