## Description

This PR contains the final bit of refactoring before I start fixing the
import problems mentioned in #5498.

The semantics of imports has so far been implemented in the `Module`
struct, and has assumed that the destination path of an import was
relative to `self`. In particular, this allows imports to any submodule
of the current module.

However, whenever that logic was triggered by the compiler the
destination path was always absolute. The only reason this worked was
that the logic was only ever triggered on the root module.

To reflect this behavior I have therefore moved the import semantics to
the `Root` struct, and made it clear in the comments that the paths are
assumed to be absolute.

This also reflects the fact that once the module structure has been
populated with the imported entities we don't actually need the import
logic anymore, so it shouldn't be located in the `Module` struct. (Name
resolution should obviously stay in the `Module` struct, so that logic
has not been moved).

I have left a couple of TODOs in the code to remind myself where I need
to make changes when I start implementing `pub use` (see #3113 ).

## 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).
- [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.

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Co-authored-by: IGI-111 <[email protected]>