Java: Rename integration test directories.

We are no longer bound to the platform-specific directories, so simplify the test organization.
If you don't want this change, just skip merging this PR. It's purely optional.

I kept the platform-specific directories around under `kotlin`,
but you could also easily merge all these together if you find them unhelpful.
I'll leave that change to you.

java/ql/integration-tests/java/multi-release-jar-java11/mod1/mod1pkg/Mod1Class.java

@@ -0,0 +1,3 @@
+package mod1pkg;
+
+public class Mod1Class { }

java/ql/integration-tests/java/multi-release-jar-java11/mod1/module-info.java

@@ -0,0 +1,5 @@
+module moduleone {
+
+  exports mod1pkg;
+
+}