fix(gatsby-transformer-remark): Revert/remark sources from different sources

[wardpeet]

Description

This change should happen in core and not in remark itself. I didn't think this through.

I should have read @m-allanson more carefully:

Thanks @alexkirsz 👍 this is a really useful feature - so much so, that something like it should be added to Gatsby core instead of this being implemented separately across different plugins.

It'll be particularly useful for the new Gatsby themes functionality that @ChristopherBiscardi is working on. As part of the themes work there's going to be an issue or rfc soon™️ (next week or after) to figure out exactly how this could be added to Gatsby core. Once the rfc has been written I'll add a link here.

You can check out more info on Gatsby themes here and here.

Related Issues

#7512