Suppot lists of WITs without requiring topological ordering

#2288 extended `Resolve::select_world` to support lists of main packages, which is used in the Rust bindings macro which accepts a list of paths, and with https://github.com/bytecodealliance/wit-bindgen/pull/1368#issuecomment-3225013168, wit-bindgen will also accept a list of paths. However, currently the paths are required to be ordered such that later packages don't depend on earlier packages. This is tolerable for some use cases, but it would be more general if we could lift this requirement.