Currently all options for inputs are totally opaque to osbuild
itself. This is neat from a seperation of concerns point of view
but has one major downside: osbuild can not verify the integrity
of the pipeline graph, i.e. if all inputs that need pipelines or
sources do indeed exists. Therefore intrdouce two generic fields
for inputs: `origin` and `references`. The former can either be
a source or a pipeline. The latter is an array of identifiers or
a dictionary where the keys are the identifiers and the values
are additional options for that id. The identifiers then refer
to either resources obtained via a source or a pipeline that has
already been built.
This represents the tree-type input, which is the simplest form
of input a stage can consume. For the stage it is just a file
system tree at a specific location and the contents itself is
mostly opaque. An obvious thing to do with such a content is
carbon copy it to a different location.
Currently this input only supports the pipelines as origins,
which are identified via their id.
