Make sure all sources are available (in particular for Wasm)

[danleh]

Double-check before the release that we have all sources available / can rebuild all Wasm line items. For one potentially because of licensing requirements, and also for reproducibility in the future.

I'm not sure whether we need to copy the sources into this repo, they could become quite large when including all transitive dependencies, e.g., for Dart or Kotlin. Maybe simply double-checking sources are available during the build, or having them as a git submodule is enough?