relative plugins from external config shared across packages breaks

Describe the bug

When you make an external config of auto to be shared across multiple packages and specify a plugin that is relative path, the plugin is not found.

To Reproduce

1. make a repository to house auto’s config
2. make a custom plugin and define it relative in package.json

"auto": {
  "plugins": [
    "./plugins/some-plugin.js",
  ]
}

1. publish this to an npm registry
2. in another initialized npm project, install the auto config repo and extend the config:

"auto": {
  "extends": "@SCOPE",
}

1. do a dry-run release of the project npx auto shipit --dry-run
2. get below error:

⚠  warning   Could not find plugin from path: $PROJECT/plugins/some-plugin.js

Expected behavior

When using auto.extend, the relative path-ed plugins from the configuration should be relative to itself and not the project that the auto config is installed into and extended in.

Additional context

I think a viable solution is to publish the plugins to npm, but that requires setting it up the auto config project to be a monorepo, and alias-ing the plugins so I can use itself to release itself.

[hipstersmoothie]

Could you test out the canary version in #1717? @sethomas

[hipstersmoothie]

This is a cool use of a shared config. gonna make this work!