Add static linter to validate Bulk Import's EE stage files

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Suggestion from !74722 (comment 741382658)

Whe adding new pipelines to:

• lib/bulk_imports/projects/stage.rb
• lib/bulk_imports/groups/stage.rb

We also need to add them to:

• ee/lib/ee/bulk_imports/projects/stage.rb
• ee/lib/ee/bulk_imports/groups/stage.rb

Which is not obvious and sometimes EE spec can not fail either, meaning that this will get merged unnoticed. We should either add a static linter to validate EE files, or only validate EE pipelines in EE specific specs, since CE pipelines are already validated in CE specs.