Refactor: Move import name resolution to Root (#5916)

## Description

Fixes #5913 .

Name resolution for imports and top-level items in module currently
happens at the module level, in part by traversing the submodules of the
current module.

This PR moves name resolution to the root of the module structure.

This refactoring is needed to enable fixes to the following issues:
- #5713 : The resolved type declaration must be understood in the
context of the declaring module rather than in the context of the
importing module, which means that name resolution must happen across
the module structure.
- #5498 : The modules from which items are imported are currently
introduced as submodules to the importing module (in addition to their
absolute location in the module hierarchy). To remove those submodules
the name resolution algorithm needs access to the full module hierarchy,
which it can get by having access to the root of the hierarchy.

## Checklist

- [x] I have linked to any relevant issues.
- [x] I have commented my code, particularly in hard-to-understand
areas.
- [x] 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](https://github.com/FuelLabs/devrel-requests/issues/new/choose)
- [x] I have added tests that prove my fix is effective or that my
feature works.
- [x] I have added (or requested a maintainer to add) the necessary
`Breaking*` or `New Feature` labels where relevant.
- [x] I have done my best to ensure that my PR adheres to [the Fuel Labs
Code Review
Standards](https://github.com/FuelLabs/rfcs/blob/master/text/code-standards/external-contributors.md).
- [x] I have requested a review from the relevant team or maintainers.

---------

Co-authored-by: IGI-111 <igi-111@protonmail.com>
Co-authored-by: Joshua Batty <joshpbatty@gmail.com>

sway-core/src/semantic_analysis/namespace/lexical_scope.rs

@@ -11,7 +11,7 @@ use crate::{
     type_system::*,
 };
 
-use super::{module::ResolvedDeclaration, TraitMap};
+use super::{root::ResolvedDeclaration, TraitMap};
 
 use sway_error::{
     error::{CompileError, StructFieldUsageContext},