Rust: Resolve Self paths in type definitions

[paldepind]

This PR:

• Implements path resolution for Self paths within type definitions.
• Slightly simplifies how Self is handled by avoiding the need for getAnItemInSelfScope. By handling Self in getAChildSuccessor it's automatically accounted for in declaresDirectly and declares.

The DCA results are uneventful.

[Copilot]

Pull request overview

Implements Rust path resolution for Self when it appears inside type definitions (e.g., in struct/enum/union field types), and updates the path-resolution tests accordingly.

Changes:

• Extend internal path-resolution to treat Self as an in-scope name for type items, similar to type parameters.
• Add new Rust test cases covering Self usage within type definitions.
• Update the expected path-resolution output to include the new module and resolutions.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
rust/ql/test/library-tests/path-resolution/path-resolution.expected	Updates expected results to reflect the new self_types test module and additional Self resolutions.
rust/ql/test/library-tests/path-resolution/main.rs	Adds self_types module with struct/enum/union cases using Self in field type definitions.
rust/ql/lib/codeql/rust/internal/PathResolution.qll	Updates internal resolution rules so Self resolves appropriately within type definitions and simplifies prior Self handling.