fix(fill): insist fill's output directory is empty or doesn't exist

[danceratopz]

🗒️ Description

Requires:

• refactor(fill): encapsulate fixture output options in FixtureOutput class #1471

Make fill's output behavior more robust: Do not write generated fixtures into an existing, non-empty output directory; it must now be empty or --clean must be used to delete it first.

Todo

• Make directory deletion/creation multip-thread/pytest-xdist friendly.

🔗 Related Issues

Fixes #1030:

• bug(fill): insist that the output directory for generated JSON fixtures is initially empty #1030

✅ Checklist

• All: Set appropriate labels for the changes.
• All: Considered squashing commits to improve commit history.
• All: Added an entry to CHANGELOG.md.
• All: Considered updating the online docs in the ./docs/ directory.