This brings the following benefits:
- Artifacts missing in the SDK tarball (such as prebuilts for linux-aarch64) can be obtained
- Artifacts can be patched more granularly (e.g. libflutter_linux_gtk is patchelf-ed for GTK3, and the linux-aarch64 assets have a postPatch to add some missing files)
- Minimal Flutter packages can be generated (e.g. mkFlutterApp only needs prebuilts for desktop Linux, and developers that don't care for desktop Linux can exclude the artifacts and dependencies)
It also paves the way for including manually built engine facts.
