Seeing this in my local console as I develop my first Netlify Build Plugin.
Seems recent and part of a netlify-build package update.

> Context
> Missing plugin
  Please use the plugin "@netlify/plugin-local-install-core" to install dependencies from the "package.json" inside your local plugins.
  Example "netlify.toml":

    package = "@netlify/plugin-local-install-core"
> Installing local plugins dependencies
   - .
> Loading plugins
   - . from undefined

I saw a few of Github issue comments, that maybe this plugin is now opt-in if necessary rather than previously opt-out?
Since it may have been opt-out, I never noticed it.

Is this really all that’s required?

package = "@netlify/plugin-local-install-core"

Hi @moop-moop,

Yes, you’re correct, that’s all that’s required. This will now be opt-out instead of opt-in.

The reason is that installing your dependencies as a first step, then installing your Functions dependencies as a second step is slower. Installing both at the same time is faster. You can do this by putting the Function’s dependencies inside the top-level package.json instead of adding a separat package.json inside the Functions directory.

However, this might not fit your preferred workflow or setup, especially regarding in a monorepo, which is why this feature is still available by opting in to this plugin.