Gastby theme/plugin issue

Bare-bones structure to demonstrate issue when a Gastby theme has a local plugin and is installed as a theme in another project/directory.

Structure based on Building a Theme tutorial, including using Yarn workspaces to manage the sibling projects, gatsby-theme-test and site.

To trigger issue:

1. run yarn workspace gatsby-theme-test to install packages
2. run yarn workspace gatsby-theme-test develop to see it successfully pass load plugins build step
3. run yarn workspace site to install packages (including gatsby-theme-test)
4. run yarn workspace site develop or cd site && gatsby develop to see it fail the load plugins build step displaying this error:

ERROR

UNHANDLED REJECTION Unable to find plugin "gatsby-local-plugin". Perhaps you need to install its package?



  Error: Unable to find plugin "gatsby-local-plugin". Perhaps you need to install its package?
  - load.js:109 resolvePlugin
    [test-gatsby-theme]/[gatsby]/dist/bootstrap/load-plugins/load.js:109:11
  ...