[Ndjson][Preparation work] Refactor import export

Based on conversation related to preparation work needed for Introducing .ndjson as a way to process import, mentioned in the comment: #197171 (comment 289977581)

As part of preparation work we should introduce Gitlab::ImportExport::Project::*, Gitlab::ImportExport::Group::* and Gitlab::ImportExport::Base::* and move files into those modules.

Related to #197171 (closed)