Compile Java and Scala sources side by side.

Sets {main,test}.scala.srcDirs such that it encompasses both Java and
Scala source files and making {main,test}.scala.srcDirs empty.  This
way, Java and Scala sources are compiled in parallel and quarkusDev
picks changes in all of them up.

Change-Id: I4fd4fab23216ee3c00ce22948ff6c971fc295410
diff --git a/build.gradle b/build.gradle
index f436299..6ddb752 100644
--- a/build.gradle
+++ b/build.gradle
@@ -5,6 +5,25 @@
     id "com.diffplug.gradle.spotless" version "3.27.0"
 }
 
+sourceSets {
+    main {
+        scala {
+            srcDirs = ['src/main/scala', 'src/main/java']
+        }
+        java {
+            srcDirs = []
+        }
+    }
+    test {
+        scala {
+            srcDirs = ['src/test/scala', 'src/test/java']
+        }
+        java {
+            srcDirs = []
+        }
+    }
+}
+
 repositories {
      mavenLocal()
      mavenCentral()