Refactor exporters to use std::ostream

For now, exporters receive the `std::string` path to output their data.

It would be useful to have them receive an `std::ostream` and let caller manage the creation of that stream.

Start by locating the exporter implementations and their callers, then trace how each currently receives its output path. The work is done when exporters receive std::ostream objects and callers manage stream creation instead of passing paths.

Written by the indexing model from the issue text.