Explicit plugin API with defined dependencies

[aaronadamsCA]

Plugin load order is currently determined entirely by array order in gatsby-config.js. If one plugin depends on another plugin, it's the user's responsibility to get the order correct.

This doesn't feel very robust. It seems the user should have better options for defining load order, and plugins should additionally have some control of this themselves.

What I'd prefer to see is something along these lines:

• If a plugin requires another plugin to be installed and executed first, it could define that plugin as a dependency.
  • And maybe if a plugin requires another plugin to be executed first only if it's already installed, it could define that plugin as an optional dependency.
• When a user adds a plugin to package.json, Gatsby automatically imports the plugin; it's no longer necessary to add it to gatsby-config.js.
  • Of course, if that plugin defines its own plugins in its own package.json, Gatsby automatically imports those, too.
• If a user requires a particular plugin execution order, they can configure their own named dependencies between plugins; otherwise Gatsby makes some basic guarantees, like that child plugins execute before parent plugins, but that's it.

This could potentially harden configurations, and allow larger projects to become much more modular.

Obviously this would need to be part of a whole new plugin API, but my hope is that we'll eventually get modern APIs that drop the need to depend exclusively on "magic exports"/shadowing (#15277 (comment)), and this could be a part of that.