Make core plugins more pluggable

[mraerino]

Which problem is this feature request solving?

When building core plugins (like https://github.com/netlify/netlify-plugin-edge-handlers) it's hard to try out changes in buildbot, because it needs these steps:

• cut a release on the plugin
• cut a release on @netlify/build
• make a PR on buildbot

Describe the solution you'd like

It might be cool to make the process for working with core plugins more flexible.

If there was a config for netlify/build in buildbot that specified the plugins to load as core plugins, buildbot could install them via its package.json and managing them would solely happen in buildbot. (This means moving the detection of whether a plugin should run into the plugin itself, which might not be easy, but doable.)

Describe alternatives you've considered

As a first step, all current core plugins could be declared peerDependencies in netlify/build and skipped if not present. That way buildbot can at least install and version them itself.

Can you submit a pull request?

n/a