Bug Report

• Yes, I reviewed the contribution guidelines.
• Yes, more specifically, I reviewed the guidelines on how to write clear bug reports.

Describe the current, buggy behavior

When the following is in the composer.json file of a WordPress project that is using Composer to manage dependencies:

"require-dev": {
	"wp-cli/wp-cli-bundle": "^2.4"
},

Running composer install with Composer version 1.10.1 generates four (4) deprecation notices saying:

Deprecation Notice: Class XXX ... does not comply with psr-0 autoloading standard. 
It will not autoload anymore in Composer v2.0.

This is problematic as we need to fail during CI/CD if Composer generates a deprecation notice because the error might be our code.

Describe how other contributors can replicate this bug

• Ensure you have Composer v1.10 or later installed.
• Run composer require-dev wp-cli/wp-cli-bundle on a project.
• Run composer install.
• See the deprecation notices in the attached screenshot.

Describe what you expect as the correct outcome

That Composer would not generate deprecation notices for WP-CLI.

Let us know what environment you are running this on

OS:	Darwin 18.7.0 Darwin Kernel Version 18.7.0: Sun Dec  1 18:59:03 PST 2019; root:xnu-4903.278.19~1/RELEASE_X86_64 x86_64
Shell:	/bin/bash
PHP binary:	/usr/local/Cellar/php/7.3.6/bin/php
PHP version:	7.3.6
php.ini used:	/usr/local/etc/php/7.3/php.ini
WP-CLI root dir:	phar://wp-cli.phar/vendor/wp-cli/wp-cli
WP-CLI vendor dir:	phar://wp-cli.phar/vendor
WP_CLI phar path:	/Users/mikeschinkel/Sites/wp-cl2.local/www/wp-content/mu-plugins/client-name/common
WP-CLI packages dir:
WP-CLI global config:
WP-CLI project config:
WP-CLI version:	2.2.0

Provide a possible solution

Rework the class hierarchy so that it is compatible with PSR-0. I would be happy to submit a patch but I assume that restructuring the directory hierarchy is something you'd want to do yourself.

Provide additional context/screenshots