Unify parsed and typed symbols namespace maps.

[tritao]

Description

As title PR, this unifies the different maps we had to represent both parsed and typed declaration for namespace module symbols.

This simplifies all the code that handles symbols since there is a single map to represent symbols at all stages in the compiler.

I tried moving all shadowing-related diagnostics to be run on the parsed declarations but it will only possible to do until the other PR for full collection/resolving is done. At the moment it leads to out of order diagnostics which breaks the test. So just left a TODO and will move those over in a separate PR once its possible to do so.

Depends on #6062.

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.

[jjcnn]

This is a very good change. I've left a single comment and a nit.