Refactor importer purlrefactor(importers): unify PURL filtering across importers

[warrior-hub]

🏆 Goal

Extract and centralize duplicated PURL filtering logic from multiple importers into a single reusable helper function.

📂 Changes

• Created a common filter_purls helper function in vulnerabilities/importers/utils.py.
• Updated all importers to use the new helper, removing duplicate PURL filtering code.
• No changes to existing behavior; return types, empty input handling, and order remain unchanged.

✅ Benefits

• Reduces code duplication across importers.
• Improves maintainability and readability.
• Prepares for future importer enhancements with a consistent PURL filtering approach.

🔒 Safety

• Functionality is identical to previous logic.
• Thoroughly tested with existing test cases to ensure behavior remains unchanged.