Don't duplicate page resource output per output format path

[bep]

This relates to #11453.

For a given output format definition you can set a path (e.g. amp) which will render the output using that as the base path.

To enable easy guessing of relative URLs of bundled resources (e.g. JSON files), we have duplicated the output per sub path. This is both a waste of resources and is rather complicated.

This setup should be rare, and is not breaking if people use Hugo's API to resolve URLs (e.g. (.Resources.Get "foo.txt").RelPermalink).