Also get sourceSets from java extension, if available

src/main/groovy/de/thetaphi/forbiddenapis/gradle/ForbiddenApisPlugin.groovy

@@ -46,15 +46,16 @@ public class ForbiddenApisPlugin extends ForbiddenApisPluginBase {
       group = JavaBasePlugin.VERIFICATION_GROUP;
     }
 
-    // retrieve Java Extension, if it is not available fallback to project convention:
+    // retrieve Java Extension and sourceSets; if not available, fallback to project convention:
     def javaExtension = project.extensions.findByName('java') ?: project
+    def sourceSets = javaExtension.hasProperty('sourceSets') ? javaExtension.sourceSets : project.sourceSets
 
     // Gradle is buggy with it's JavaVersion enum: We use majorVersion property before Java 11 (6,7,8,9,10) and for later we use toString() to be future-proof:
     Closure targetCompatibilityGetter = { (javaExtension.targetCompatibility?.hasProperty('java11Compatible') && javaExtension.targetCompatibility?.java11Compatible) ?
         javaExtension.targetCompatibility.toString() : javaExtension.targetCompatibility?.majorVersion };
 
     // Define our tasks (one for each SourceSet):
-    project.sourceSets.all{ sourceSet ->
+    sourceSets.all{ sourceSet ->
       String sourceSetTaskName = sourceSet.getTaskName(FORBIDDEN_APIS_TASK_NAME, null);
       def sourceSetTask = TASK_AVOIDANCE_AVAILABLE ? project.tasks.register(sourceSetTaskName, CheckForbiddenApis.class) :
               project.tasks.create(sourceSetTaskName, CheckForbiddenApis.class);