refactor: remove parallel walk

[knqyf263]

Description

Since filepath.WalkDir is fast enough now, this PR removes github.com/saracen/walker. Also, it defines the Walker interface so that other tools can override the behavior.

Related issues

• Close https://github.com/aquasecurity/trivy/issues/5179

Checklist

• [x] I've read the guidelines for contributing to this repository.
• [x] I've followed the conventions in the PR title.
• [x] I've added tests that prove my fix is effective or that my feature works.
• [ ] I've updated the documentation with the relevant information (if needed).
• [ ] I've added usage information (if the PR introduces new options)
• [ ] I've included a "before" and "after" example to the description (if the PR is a user interface change).

[knqyf263]

@santhosh1729 I think we'll merge this PR now, and you can give us feedback anytime.

[github-actions[bot]]

This PR is stale because it has been labeled with inactivity.

[knqyf263]

@DmitriyLewen Could you quickly take a look? Aqua confirmed it, and it should be fine, but I want to double-check it.