Fix golint to check all project files recursively

What does this MR do?

Prior to this lint_code job has checked only the root dir of the project. This MR enables it to check all project files recursively by running golint -set_exit_status ./.... And also contains fixes for all golint complaints.

What are the relevant issue numbers?

gitlab-org/gitlab#14026 (closed)

Does this MR meet the acceptance criteria?

• [-] Changelog entry added
• [-] Documentation created/updated for GitLab EE, if necessary
• [-] Documentation created/updated for this project, if necessary
• [-] Documentation reviewed by technical writer or follow-up review issue created
• [-] Tests added for this feature/bug
• [-] Job definition updated, if necessary
  • [-] Auto-DevOps template
  • [-] Job definition example
  • [-] CI Templates
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer