Tidy up gradle source sets.

This replaces the default source directories (src/main/java, etc)
with our own, instead of just appending ours to the list.

build.gradle

@@ -19,20 +19,12 @@ subprojects {
 
     sourceSets {
         main {
-            java {
-                srcDir 'src'
-            }
-            resources {
-                srcDir 'res'
-            }
+            java.srcDirs = ['src']
+            resources.srcDirs = ['res']
         }
         test {
-            java {
-                srcDir 'test'
-            }
-            resources {
-                srcDir 'test-res'
-            }
+            java.srcDirs = ['test']
+            resources.srcDirs = ['test-res']
         }
     }