Add support for foramt version 2. This is slightly easier than
version 1, since there is no recursion. In addition to the
path of the manifest to import the pipeline from, the pipeline
identifier needs to be specified.
Source merging is also different since in format version two,
there is a generic "items" key, which means we can merge
other sources than "org.osbuild.files".
This adds a new MPP which supports importing pipelines from another
file. It simply looks for "mpp-import-pipeline" tags at the same
position where we would expect a "pipeline" tag. It then uses the "path"
attribute in it to find a manifest. From this manifest, the "sources"
are merged back into the original sources, and the "pipeline" is taken
verbatim to replace the "mpp-import-pipeline".
The idea is to allow importing build-pipelines from other files into our
test manifests, without duplicating the build-pipeline everywhere.
