Flatten the datastructure for external dependencies in the namespace module

[jjcnn]

Description

Fixes #6838 .

Status:

• Introduced Project struct
• Moved externals from Package to Project
• Introduce mapping from Forc.toml dependencies section to Package
• Updated most access functions in namespace.rs, package.rs, project.rs

TODO:

• Fix commented-out functions in namespace.rs
• Update contract_helpers.rs
• Update calls to functions in namespace module
• Update forc-pkg to create and supply Project struct instead of Package struct
• Remove Clone attribute from Package and Module, possibly TraitMap
• Update sway-lsp and other tooling to use Project instead of Package

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.