[Feat]: Provide semantic item and type information for dependencies

[xFrednet]

Summary

Rustc splits the compilation of a project into multiple compilation units, called crates. This means, that at a given time, rustc only has access to the AST of a single crate. Therefore, it can happen that the AST representation of a node is unavailable, even if the ID is valid. Due to the fact, that the item comes from a dependency.

Even if there is no AST representation, there is usually some semantic information available. This is required for type checking and name resolution, among other things. There are a few key differences, which prevent Marker from turning this semantic information into an AST representation. For example, there are no spans or bodies. The information how an item was created is simply lost, as it's not required for dependents.

This issue suggests the addition of semantic item and type information. Every valid ItemId or TyDefId should have valid semantic information available, even if the AST node isn't available. This concept also extends to other AST nodes like EnumVariants etc.