Checkstyle: Better default behavior for multiprojects

When the checkstyle plugin is used in a multiproject, it should know to search in the parent project’s config/checkstyle/{checkstyle,suppressions}.xml. Currently, gradle check in a subproject errors with a message about missing configuration.

As a workaround, users can do something like this in the parent build.gradle:

allprojects {
  apply plugin: 'checkstyle'

  checkstyle {
    configFile = "${rootProject.projectDir}/config/checkstyle/checkstyle.xml" as File
    configProperties = [configDir: "${rootProject.projectDir}/config"]

But it would be nice if the checkstyle plugin simply behaved like this out of the box.