Lazily register sonar and sonarqube tasks

[GreyTeardrop]

Replace eager task registration via TaskContainer.create() with lazy registration via TaskContainer.register().

Motivation: analyzing a build scan, I've noticed that my build had 2 tasks created immediately. Those turned out to be sonar and sonarqube.

It would be great to avoid creating those tasks if SonarScanner isn't executed.

[pwagland]

Changes look good to me, and saves small amount of time per project. When you have hundreds or thousands to project these small bits add up. The tasks are correctly configured lazily.

[dorian-burihabwa-sonarsource]

We need to check that the API is compatible with the lowest version of Gradle we support 5.6.4

[GabrielFleischer]

A ticket has been created, it will be taken care of in a future sprint.

https://sonarsource.atlassian.net/browse/SCANGRADLE-158

[GreyTeardrop]

We need to check that the API is compatible with the lowest version of Gradle we support 5.6.4

As far as I can see, configuration avoidance methods including TaskContainer.register() were first introduced in Gradle 4.9 and made stable in Gradle 5.1.