All sources fetch various types of `items`, the specific nature
of which is dependent on the source type, but they are all
identifyable by a opaque identifier. In order for osbuild to
check that all the inputs that a stage needs are are indeed
contained in the manifest description, osbuild must learn what
ids are fetched by what source. This is done by standarzing
the common "items" part, i.e. the "id" -> "options for that id"
mapping that is common to all sources.
For the version 1 of the format, extract the files and ostree
the item information from the respective options.
Adapt the sources (files, ostree) so that they use the new items
information, but also fall back to the old style; the latter is
needed since the sources tests still uses the SourceServer.
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