LiteParsedDocumentSource::getMutableSubpipelines() exposes the underlying vector of subpipelines during desugaring. The desugarer doesn't need to mutate the vector itself, instead it just needs to desugar each individual subpipeline. The order of the vector container must remain immutable. As part of this ticket, implement a container that allows the desugarer to expand each subpipeline, while guaranteeing the subpipeline vector itself remains unchanged.