This issue is part 1/2 for setting up the new infrastructure for developing standalone plugins.

Requirements

This work should be implemented against a new feature/modules-to-plugins feature branch.

• Create new plugins folder in the repository root.
• Add a .gitkeep to it.
• Add the plugins folder to be ignored in .gitattributes so that it won't be part of the Performance Lab plugin build.
• Restructure the plugins.json file:
  • Going forward, there should be always two top level properties: modules and plugins.
  • modules should have the map of "module => plugin data (slug and version)", i.e. the same thing that the file contains today.
  • plugins should have an array of plugin slugs.
• Update the existing workflows that use the plugins.json file to look for modules within the new modules property in the file.
• Add references to the new plugins folder in any dev tooling that currently lints/covers the modules folder (e.g. in phpcs.xml.dist etc.).

Note: Additional changes to build/publish the new plugins should be implemented in a separate follow-up issue.