Namespace module cleanups

[tritao]

Description

This PR represents another round of cleanups, this time mostly around Namespace and Root.

Checklist

• I have linked to any relevant issues.
• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation where relevant (API docs, the reference, and the Sway book).
  • If my change requires substantial documentation changes, I have requested support from the DevRel team
• I have added tests that prove my fix is effective or that my feature works.
• I have added (or requested a maintainer to add) the necessary Breaking* or New Feature labels where relevant.
• I have done my best to ensure that my PR adheres to the Fuel Labs Code Review Standards.
• I have requested a review from the relevant team or maintainers.

[codspeed-hq]

CodSpeed Performance Report

Merging #6660 will improve performances by 10.04%

_{Comparing tritao:namespace-module-cleanups (ff9dd31) with master (d6b9def)}

Summary

⚡ 1 improvements
✅ 21 untouched benchmarks

Benchmarks breakdown

	Benchmark	master	tritao:namespace-module-cleanups	Change
⚡	document_symbol	5.2 ms	4.7 ms	+10.04%

[jjcnn]

I'm good with the changes to type_resolve.rs, but I would really like if the code in namespace.rs and root.rs wasn't moved around too much. I'm making a lot of changes in those files, so it's super complicated to keep track of my changes when code is moved to somewhere else.

[tritao]

Updated the PR, I've removed most of the bigger changes to Namespace and Root now, we can re-access if they are still needed once the dust settles.