Replace custom lazy loading with lazy_loader package

[hakonanes]

Description of the change

• Replace custom lazy loading of modules with the lazy_loader package: https://scientific-python.org/specs/spec-0001/#lazy_loader. This should reduce import times.
• Use absolute internal imports everywhere but the stub files __init__.pyi
• Move checks for whether optional dependencies are installed to a private kikuchipy.constants module
• Create a private logging module (everything not listed in API reference in docs is private)

Progress of the PR

• Docstrings for all functions
• Unit tests with pytest for all lines
• Clean code style by running black via pre-commit

For reviewers

• The PR title is short, concise, and will make sense 1 year later.
• New functions are imported in corresponding __init__.py.
• New features, API changes, and deprecations are mentioned in the unreleased
  section in CHANGELOG.rst.
• New contributors are added to kikuchipy/__init__.py and .zenodo.json.