A manifest is simply a struct containing a sources and a pipeline
object. We want to store and transfer pielines always with their
sources, and will use the manifest for this.
When serialized, a manifest can be the input to `osbuild`, just
like a bare pipeline can be. This means there will be no need
to pass in sources separately on the commandline.
Signed-off-by: Tom Gundersen <teg@jklm.no>
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