Refactor import_export

Nikola Milojevicrequested to merge
refactor-modules-for-import-export into master

What does this MR do?

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

This MR, tries to introduce Gitlab::ImportExport::Project::*, Gitlab::ImportExport::Group::* and Gitlab::ImportExport::Base::* and move files into those modules.

Closes #207846 (closed)

Related to #197171 (closed)

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