Ignore line extraction on historic scans

What does this MR do?

This MR fixes the bug described here: gitlab-org/gitlab#223565 (closed). SECRET_DETECTION_HISTORIC_SCAN was causing the analyzer to fail due to the scanners trying to extract the line number from non-existing files. This bug arises when a secret is detected somewhere in the git history, but later the file that contains that secret is deleted. This MR adds some logic to prevent that from happening.

I also did a little refactoring to consolidate some items into the scanner/scanner.go file.

What are the relevant issue numbers?

gitlab-org/gitlab#223565 (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
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• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer