For serializeation, make our own private structs. The structs
in the target package are not exactly the same as the ones used by
weldr, so in order to avoid too many compromises, let's just do
an explicity translation.
As a general principle, we aim to only use private types for
serialization and rather translate than reuse for different
purposes.
Signed-off-by: Tom Gundersen <teg@jklm.no>