Provide configDirectory property for Checkstyle plugin

[baev]

This property is required in order to have config_loc variable set, for example to use it with SuppressionFilter module:

<module name="SuppressionFilter">
    <property name="file" value="${config_loc}/suppressions.xml"/>
</module>

https://docs.gradle.org/current/dsl/org.gradle.api.plugins.quality.CheckstyleExtension.html#org.gradle.api.plugins.quality.CheckstyleExtension:configDirectory

FYI: as workaround I use afterEvaluate to pass configDirectory to Checkstyle and it works just fine

[xvik]

checkstyle.configDirectory set to gradle/config/checkstyle/ (by default)

Also, suppressions filter added to the default file:

<module name="SuppressionFilter">
        <property name="file" value="${config_loc}/suppressions.xml"/>
        <property name="optional" value="true"/>
</module>

(with optional=true to ignore absent file)

So it would be enough now to just drop custom suppressions.xml into gradle/config/checkstyle/ and it would be used automatically.