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ARROW-17106: [Python] Move init code to core and expose only API (#13802)

Will close [ARROW-17106](https://issues.apache.org/jira/browse/ARROW-17106)

Moves all current code in `pyarrow/parquet/__init__.py` to `pyarrow/parquet/core.py` and re-exports the public API from core.py inside of the init file.

Authored-by: Miles Granger <miles59923@gmail.com>
Signed-off-by: Antoine Pitrou <pitrou@free.fr>