[Backport] magento/magento2#?: fix issue with di.xml config merging for classes array argument in case of different sources and the class name difference in leading slash.

[swnsma]

Description (*)

See original PR #20891

Fixed Issues (if relevant)

1. Magento\Catalog\Api\ProductRenderListInterface returns products regardless of visibility #20409
2. Types not merged for etc/di.xml etc/adminhtml/di.xml #21193

Manual testing scenarios

Pre-Conditions

1. Change constructor of \Magento\Cms\Controller\Index\Index and add new argument 'data' (simple way to visualize it):

public function __construct(`
`        Context $context,`
        ForwardFactory $resultForwardFactory,
        ScopeConfigInterface $scopeConfig = null,
        Page $page = null,
        array $data = []
    ) {
        var_dump($data);
        die;
    }

Steps to reproduce

1. Create module A and B with simple config set (registration.php and module.xml).
2. Execute in terminal php bin/magento setup:upgrade.
3. Create etc/di.xml for module A:

<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    <type name="Magento\Cms\Controller\Index\Index">
        <arguments>
            <argument name="data" xsi:type="array">
                <item name="p1" xsi:type="string">Hello Phrase1</item>
            </argument>
        </arguments>
    </type>
</config>

4. Create etc/di.xml for module B:

<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    <type name="\Magento\Cms\Controller\Index\Index">
        <arguments>
            <argument name="data" xsi:type="array">
                <item name="p2" xsi:type="string">Hello Phrase2</item>
            </argument>
        </arguments>
    </type>
</config>

5. Execute in terminal php bin/magento cache:flush
6. Open home page.

Expected results

Actual results

Contribution checklist (*)

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

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