Avoid duplication of types when using generated stubs

[vigoo]

When having a circular reference through generated stubs, some data types are getting inlined in the generated dependency-WITs because the stub WITs cannot refer to the top level package.

To avoid this, we can change how stub dependencies are added to a project, by doing a one-time transformation of the caller project's WIT directory, introducing a new top-level WIT package and moving the original user defined one to the wit/deps dependencies. This way we don't have to inline the user-defined types in the generated stubs.