Add a virtual package

[mcaskill]

• I've read the guidelines for Contributing to Roots Projects
• This request isn't a duplicate of an existing request
• This is not a personal support request that should be posted on the Roots Discourse community

Summary

Add a provide section with wordpress/core-implementation as a virtual package to all builds of WordPress.

Example:

"provide": {
    "wordpress/core-implementation": "5.4.1"
}

Motivation

This can be useful for WordPress plugins, themes, and libraries that are managed by Composer.

A library, such as johnbillion/extended-cpts, could list the WordPress versions that conflict that package:

"conflict": {
    "wordpress/core-implementation": "<4.8"
}

A type of boilerplate could even require WordPress:

"require": {
    "php": ">=7.1",
    "wordpress/core-implementation": ">=5.0"
}

Additional context

John Bloch implemented this feature a few years ago (johnpbloch/build-wp#3)

[swalkinshaw]

Closing in favour of #22