refactor: split export_apkgs.py into a module

Dockerfile

export_apkgs/__init__.py

export_apkgs/__main__.py

export_apkgs/config.py

export_apkgs/error.py

export_apkgs/exporter.py

export_apkgs/format.py

export_apkgs/model.py

test/fixtures/resource_missing/.apkg-spec.yaml

test/fixtures/resource_missing/content.csv

test/test_export_apkgs.py