Add support to allow partial use rules per namespace

[Toflar]

Implements feature request in #1310

It adds namespace-aware control to SlevomatCodingStandard.Namespaces.ReferenceUsedNamesOnly partial-use handling.

As already explained in the original issue, the reason why I would like to have this is that some codebases intentionally use partial references for selected packages, especially Doctrine ORM and Symfony Validator.

You can now either just allow them:

<rule ref="SlevomatCodingStandard.Namespaces.ReferenceUsedNamesOnly">
	<properties>
		<property name="allowPartialUses" value="false"/>
		<property name="namespacesAllowedToUsePartially" type="array">
			<element value="Doctrine\ORM\Mapping as ORM"/>
			<element value="Symfony\Component\Validator\Constraints as Assert"/>
		</property>
	</properties>
</rule>

or even enforce them:

<rule ref="SlevomatCodingStandard.Namespaces.ReferenceUsedNamesOnly">
	<properties>
		<property name="allowPartialUses" value="false"/>
		<property name="namespacesRequiredToUsePartially" type="array">
			<element value="Doctrine\ORM\Mapping as ORM"/>
			<element value="Symfony\Component\Validator\Constraints as Assert"/>
		</property>
	</properties>
</rule>

[Toflar]

Updated the PR so it now also allows namespace aliases properly supporting the Symfony\Component\Validator\Constraints as Assert as well. Initial PR description updated 😎

[kukulich]

@Toflar Looks good. Can you please squash it to one commit?

[Toflar]

Done 😊