[Python] Move parquet code from __init__.py and expose only API

[asfimport]

As discussed on #12837 (comment)

it would be better to move all code to pyarrow/parquet/core.py and just import the APIs from the _init_{_}.py{_} instead of having all the parquet pyarrow code on the {_}init{_}_.py file.

Reporter: Raúl Cumplido / @raulcd
Assignee: Miles Granger / @milesgranger

Related issues:

• [Python] Refine pyarrow.parquet API exposure (is related to)

PRs and other links:

• GitHub Pull Request #13802

_{Note: This issue was originally created as ARROW-17106. Please see the migration documentation for further details.}

[asfimport]

Antoine Pitrou / @pitrou:
Issue resolved by pull request 13802
#13802