Add a new method that will copy the downloaded data for a specified
source to a directory, creating a directory structure so that the
target directory can then be used to initialize the osbuild cache,
i.e. the target directory be used for the `cache_from` parameter in
the `test.OSBuild` constructor.
The main reason why this is done per source, and not for all sources,
is that not all source can be copied via a plain `cp` operation or
easily added to, in case that the `target` directory already contains
data. The `org.osbuild.ostree` source is such an example.
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