Refactoring howl/data dir

[ljj7975]

#65
reorganize the files under howl/data
I have split some files and grouped some relevant files
formatting handled by pre-commit

There was no code change other than some renaming

Since this change is made upon #83, and the change is quite huge
so please review #83 and this one individually

The files are organized as follow

├── common
│   ├── batch.py
│   ├── example.py
│   ├── frame.py
│   ├── __init__.py
│   ├── labeler.py
│   ├── metadata.py
│   ├── phone.py
│   ├── searcher.py
│   ├── tokenizer.py
│   └── vocab.py
├── dataloader.py
├── dataset
│   ├── common_voice_dataset_loader.py
│   ├── dataset_loader.py
│   ├── dataset.py
│   ├── dataset_writer.py
│   ├── gsc_dataset_loader.py
│   ├── hey_snips_dataset_loader.py
│   └── __init__.py
├── __init__.py
├── stitcher.py
└── transform
    ├── batchifier.py
    ├── __init__.py
    ├── meyda.py
    ├── operator.py
    └── transform.py