Modernize project and fix URL resolution bugs

[google-labs-jules]

Evaluated the repository and performed several improvements:

1. Modernized the project by adding Go modules support and fixing outdated/broken import paths.
2. Fixed a critical bug in Link.Url() where it was incorrectly assuming the 'href' attribute position and was mutating the base URL during resolution.
3. Enhanced robustness by adding proper error handling to NewCrawler and its callers.
4. Simplified Client initialization to be more idiomatic.
5. Added a new test suite (scour_test.go) covering various URL resolution scenarios and ensuring the fixes work as intended.

---

PR created automatically by Jules for task 9049834780028524643 started by @brettfxio

[google-labs-jules]

👋 Jules, reporting for duty! I'm here to lend a hand with this pull request.

When you start a review, I'll add a 👀 emoji to each comment to let you know I've read it. I'll focus on feedback directed at me and will do my best to stay out of conversations between you and other bots or reviewers to keep the noise down.

I'll push a commit with your requested changes shortly after. Please note there might be a delay between these steps, but rest assured I'm on the job!

For more direct control, you can switch me to Reactive Mode. When this mode is on, I will only act on comments where you specifically mention me with @jules. You can find this option in the Pull Request section of your global Jules UI settings. You can always switch back!

New to Jules? Learn more at jules.google/docs.

---

For security, I will only act on instructions from the user who triggered this task.