Do not parallelize definition building if it gives no advantage

For the cases when there are 1 or 2 cores the fork join pool is of size 1 and therefore does not achieve parallelized execution. For this case we can load definitions on the main thread.

core/src/main/java/io/micronaut/core/io/service/ServiceScanner.java

@@ -239,7 +239,7 @@ public void collect(Collection<S> values) {
 
         @Override
         public void collect(Collection<S> values, boolean allowFork) {
-            if (allowFork) {
+            if (allowFork && ForkJoinPool.getCommonPoolParallelism() > 1) {
                 ForkJoinPool.commonPool().invoke(this);
                 for (RecursiveActionValuesCollector<S> task : tasks) {
                     task.join();