Instead of having build pipelines nested within the pipeline it is
the build pipeline for, the nested structure is transferred into a
flat list of pipelines. As a result the recursion is gone and all
the pipelines and trees are build one after the other. This is now
possible since floating objects are kept alive by the store itself
and all trees that are being built are transparently via them.
The immediate result dictionary changed accordingly. To keep the
JSON output of osbuild the same, the result is now routed through
a format specific converter.
Additionally, the v1 format module gained a function to retrieve
the global tree_id and output_id. With the new models those global
ids will go away eventually and thus need to go through the format
specific code.
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