Remove DeRef and DeRefMut implementations in the namespace module

[jjcnn]

Description

This PR is the first step of a large refactoring of the namespace module.

The PR removes the DeRef and DeRefMut implementations in the namespace module, as they make refactoring more difficult to keep track of. This change means that calls to Root and Module must now explicitly go through Namespace, since that is the type that is passed along during typechecking.

I'm submitting the PR now because there have been changes to master while I've been working on this, and the changes are really difficult to merge, so I want master to be relatively stable.

Subsequent PRs will:

• Eliminate the dst parameter from all import functions (the destination is always the current module).
• Move all import functions to Namespace (they currently reside in Module where they aren't needed, and where they require a clone the module path to work).
• Move Namespace::init to Root, and change how it is used (it currently holds a constant declaration of CONTRACT_ID for contract modules, and that declaration is cloned into every submodule. The nice way to do this is to declare it in the root module and implicitly import it into each submodule).
• Eliminate the Option part of Module::name (modules are not allowed to be nameless - the option only exists because they root module can in principle be nameless, but it ought to be named based on the package the module structure is in).
• Fix the import and name resolution functions so that they don't treat absolute paths as relative (this should fix some of the performance issues we're seeing).
• Make Namespace::root a reference so that the entire module structure isn't cloned for every submodule (this requires a change in ownership of the root module).
• Introduce reexports in the form of pub use.

This PR is the first step in fixing #5498.

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).
• 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.