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.
