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/kotlin/all-platforms/enhanced-nullability/NotNull.java

@@ -0,0 +1,6 @@
+package org.jetbrains.annotations;
+import java.lang.annotation.*;
+
+// Stub of @NotNull:
+@Target({ElementType.METHOD, ElementType.FIELD, ElementType.PARAMETER, ElementType.LOCAL_VARIABLE, ElementType.TYPE_USE})
+public @interface NotNull { }