PageModelApiPipeline should not be possible to be overridden on sitemap item level

The problem is as follows:

The SpaSitePipeline can be set on individual sitemap items. This way we can support hybrid channels: Partly normal pages, partly SPA.

When accessing however the sitemap items with SpaSitePipeline via the mount that has 'PageModelApiPipeline', the pipeline name should not be overridden to 'SpaSitePipeline'.

We need to come up with a solution for this. Perhaps on the resourceapi mount a property

@hst:finalPipeline = true 

or some other solution.