Don't let manifesto mutate the manifest json.

[cbeer]

This is the other part of #4016. manifesto's been mutating the provided manifest to add its data structures. This isn't a big problem for normal manifests, but the collection representation has a cyclic reference.

[marlo-longley]

This works locally for me.

My only suggestion is to add a comment otherwise this could be a tad cryptic.
Something like ... "Use JSON stringify/parse to create a deep copy and prevent Manifesto from mutating the json" ?