Avoid resolving dependencies at configuration time

[jdai8]

Fixes #263.

In most cases, checking the dependency tree is not necessary, as shouldRunAfter does not add any tasks to the task graph. I think it's simpler to add the task ordering to all classes tasks in the project.

However, this does change the behavior in certain cases. For example, if someone runs ./gradlew classes composeUp with no isRequiredBy, currently the plugin will have no ordering between the two tasks. These use cases seem unlikely to me though.

I can change the implementation to use taskGraph.whenReady if you think that's better. That implementation is just harder to unit test, as it relies on Gradle's API.

[augi]

Hello, thank you for this PR!

I wasn't sure what Gradle does if dependsOn and shouldRunAfter are in conflict. But it looks like it is OK, probably because shouldRunAfter is just a kind of recommendation.

task taskA { doLast { logger.lifecycle('A') } }
task taskB { doLast { logger.lifecycle('B') } }
tasks.taskA.dependsOn tasks.taskB
tasks.taskB.shouldRunAfter tasks.taskA