composer-installer: Incorrect relative paths for WPCS

Problem/Motivation

Possibly related to #32.

With the following in my composer:

"dealerdirect/phpcodesniffer-composer-installer": "^0.4",
"wimg/php-compatibility": "dev-feature/prevent-conflicts-with-other-standards",
"wp-coding-standards/wpcs": "dev-develop"

the installed_paths phpcs config value is wrong.

Composer resolves this as using PHPCS 3.0.2, and the relative branches of WPCS and PHPCompatibility, both of which support PHPCS 3.*.

Expected behaviour

I expect the paths to be correct. With 0.3 of this plugin. I get:

<?php
 $phpCodeSnifferConfig = array (
  'installed_paths' => '/Users/gary/Local Sites/incipio/app/public/wp-content/themes/incipio/vendor/wp-coding-standards/wpcs,/Users/gary/Local Sites/incipio/app/public/wp-content/themes/incipio/vendor/wimg/php-compatibility',
)
?>

phpcs -i shows all of the standards correctly available, and phpcs runs correctly against the phpcs.xml.dist in my project.

Actual behaviour

With 0.4 of the plugin, I get:

<?php
 $phpCodeSnifferConfig = array (
  'installed_paths' => '../../wp-coding-standards/wpcs/WordPress/,../../wp-coding-standards/wpcs/WordPress-Core/,../../wp-coding-standards/wpcs/WordPress-Docs/,../../wp-coding-standards/wpcs/WordPress-Extra/,../../wp-coding-standards/wpcs/WordPress-VIP/,../../wimg/php-compatibility/PHPCompatibility/',
)
?>

phpcs -i still does show the right standards are available, but, phpcs does not run correctly. Depending on the first called ruleset in phpcs.xml.dist, I get an error the form:

ERROR: Referenced sniff "WordPress" does not exist

This also happens if phpcs . --standard=WordPress is called.

Interestingly, calling phpcs . --standard=WordPress-Core does appear to run.

I’m not sure where the issue lies exactly, but @jrfnl says:

PHPCS 3.x is supposed to support standards as a top-level folder (as in how PHPCompatibility used to be layed out and how DealerDirect registered the paths now), but that looks to be buggy based on your findings.

I can run some tests with that and will report my finding to PHPCS, but basically as long as the installed_paths are registered in the PHPCS 2.x way (which is also still the recommendation for PHPCS 3.x), things should be fine.