[Splink 4] Move key user-facing classes/functions to `__init__.py`? about splink HOT 1 OPEN

def __getattr__(name):
    if name == "SparkAPI":
        try:
            from splink.spark.database_api import SparkAPI

            return SparkAPI
        except ImportError:
            raise ImportError(
                "SparkAPI cannot be imported because its dependencies are not "
                "installed. Please `pip install pyspark`."
            )
    raise AttributeError(f"module 'splink' has no attribute '{name}'")