hugolib: Do not create paginator pages for the other output formats

hugolib: Do not create paginator pages for the other output formats

This is a recent regression in Hugo, where we have started to produce `/page/30/index.json` when the main output format (usually `HTML`) is set up with pagination.

For JSON this is potentially lot of superflous work and hurts performance.

This commit reinstates the earlier behaviour: We only create paginators if in use in the main output format.

And add a test for it to prevent this from happening again.

Fixes #4890