Move org.gradle.api.sonar.runner -> org.gradle.sonar.runner.

+review REVIEW-5188
It's not XML, it's text.

Why have we lived with this for so long?
Impose 'tasks' and 'plugins' packages conventions.

+review REVIEW-5188
Convert task implementation to Java.

+review REVIEW-5188
Convert plugin class to Java.

Highlighted some design issues that will be fixed in a subsequent change.

+review REVIEW-5188
Move application of sonarRunner {} actions out of the extension, to avoid exposing infrastructure methods to users.

This complicates the design somewhat, but makes things simpler for users by not exposing them to methods they shouldn't call.

To take this further, we could turn the extensions into interfaces.

+review REVIEW-5188