Resolve "Follow-up from "Export rake task""

This MR is moving classes from import.rake and export.rake tasks to proper lib folder in lib/gitlab/import_export/project.

• It should remove requirements inside rake task
• It should remove need to invoke rake task inside specs
• It should speed up specs execution time

This is related to follow up issue: #207986 (closed)

Closes #207986 (closed)