Skip remediation when yarn lock file unchanged

What does this MR do?

• skip remediation when yarn lock file unchanged
• improve log messages, and log error messages when CLI commands fail
• refactor code, rename variables and functions

What are the relevant issue numbers?

gitlab-org/gitlab#220487 (comment 468273491)

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
  • see new dedicated QA job in CI config; it doesn't check the output but ensures that it works, and makes it possible to check the debug messages
  • see discussion that proves that this works, even though it's very limited
• 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