Clean up dependencies/peer dependencies for image plugins

[jlengstorf]

Looking deeper into #3545, it looks like things are a little weirder than I initially thought.

I do have gatsby-plugin-sharp installed as a dependency in my site, but not included in the plugin array inside gatsby-config.js.

My guess is that this doesn't fail because gatsby-transformer-sharp is directly importing what it needs, as is gatsby-remark-images.

Currently, gatsby-plugin-sharp is a direct dependency of gatsby-remark-images and not referenced at all in gatsby-transformer-sharp.

To experiment with this, I installed gatsby-plugin-sharp but did not add it as a plugin to Gatsby, then added gatsby-remark-images as a plugin and everything worked as expected.

So this poses two questions:

1. Is gatsby-plugin-sharp actually a plugin? Or is it a utility package that other plugins rely on?
2. Given that both gatsby-transformer-sharp and gatsby-remark-images work without gatsby-plugin-sharp being added as a plugin, would it make more sense to add it as a dependency to both for an easier setup?

[jlengstorf]

Had an offline conversation with @KyleAMathews that clarified this.

• gatsby-plugin-sharp manages a job queue that tells the Gatsby bootstrap process when it's complete
• It's possible to run without this reporting, but likely that problems will occur at some point.
• There is an idea (see Add a generic jobs logging framework to the Gatsby core to allow custom plugin logging #4299) to move this job queue/reporting into the core, which would eliminate the dependency on gatsby-plugin-sharp for other plugins — instead, the logic would be moved into a utility package (e.g. gatsby-sharp) that would not need to be added as a Gatsby plugin.
• For now, gatsby-plugin-sharp should be a peerDependency of both gatsby-remark-images and gatsby-transformer-sharp and MUST be declared in the gatsby-config.js plugin array.

I'll submit a PR for this shortly.