Allow custom config files with virtual types only by adding generic schema locator

[Vinai]

Using custom configuration files requires a number of class implementations.
Most of these can be satisfied by using generic implementations found within the Magento\Framework\Config namespace and injectable parameters.

• Magento\Framework\Config\DataInterface => Magento\Framework\Config\Data
• Magento\Framework\Config\ReaderInterface => Magento\Framework\Config\Reader\Filesystem
• Magento\Framework\Config\ConverterInterface => Magento\Framework\Config\Converter\Dom

The one that is missing is a generic implementation of the schema locator interface Magento\Framework\Config\SchemaLocatorInterface
The implementation always is very simple, it only returns the names of the schema files. Not a lot of work but just another bit of annoying boilerplate to write.

This PR adds a generic schema locator implementation.
Here is an example di.xml how that might be used:

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xsi:noNamespaceSchemaLocation="../../../../../../lib/internal/Magento/Framework/ObjectManager/etc/config.xsd">
    <type name="Example\DiConfig\Controller\Index\Index">
        <arguments>
            <argument name="config" xsi:type="object">Example\DiConfig\Model\ConfigData\Virtual</argument>
        </arguments>
    </type>
    <virtualType name="Example\DiConfig\Model\ConfigData\Virtual"
                 type="Magento\Framework\Config\Data">
        <arguments>
            <argument name="reader" xsi:type="object">Example\DiConfig\Model\Config\Reader\Virtual</argument>
            <argument name="cacheId" xsi:type="string">example_config</argument>
        </arguments>
    </virtualType>
    <virtualType name="Example\DiConfig\Model\Config\Reader\Virtual"
                 type="Magento\Framework\Config\Reader\Filesystem">
        <arguments>
            <argument name="converter" xsi:type="object">Example\DiConfig\Model\Config\Converter\Virtual</argument>
            <argument name="schemaLocator" xsi:type="object">Example\DiConfig\Model\Config\SchemaLocator\Virtual</argument>
            <argument name="fileName" xsi:type="string">example.xml</argument>
        </arguments>
    </virtualType>

    <virtualType name="Example\DiConfig\Model\Config\Converter\Virtual"
                 type="Magento\Framework\Config\Converter\Dom"/>

    <virtualType name="Example\DiConfig\Model\Config\SchemaLocator\Virtual"
                 type="Magento\Framework\Config\GenericSchemaLocator">
        <arguments>
            <argument name="moduleName" xsi:type="string">Example_DiConfig</argument>
            <argument name="schema" xsi:type="string">example.xsd</argument>
        </arguments>
    </virtualType>
</config>

No custom class implementations are required anymore to use custom configuration files. Of course they still should be used if it makes sense though, mostly for the DataInterface to provide readable getters.

[piotrekkaminski]

Thanks Vinai!

[vrann]

@Vinai thanks, this might be helpful feature.