[reporting] render unused rule-scoped skips as nested list

[TomasVotruba]

Renders unused rule-scoped skips as a nested list instead of inline rule => path / rule => [ ... ] form.

Before:

 * Rector\...\ReturnTypeFromReturnDirectArrayRector => [ app/.../LeadModel.php
    app/.../TranslationEntityTrait.php ]

After:

 * Rector\...\ReturnTypeFromReturnDirectArrayRector:
     * app/bundles/LeadBundle/Model/LeadModel.php
     * app/bundles/CoreBundle/Entity/TranslationEntityTrait.php

Single and multiple paths now use the same nested shape via SymfonyStyle::listing().