GH-787 - Allow references between sibling sub-modules.

Previously, we rejected references between sub-modules both contained in the same parent package.

Related ticket: GH-578.

spring-modulith-core/src/test/java/example/ni/nested/b/second/InNestedBSecond.java

@@ -16,10 +16,12 @@
 package example.ni.nested.b.second;
 
 import example.ni.nested.InNested;
+import example.ni.nested.b.first.InNestedBFirst;
 
 /**
  * @author Oliver Drotbohm
  */
 public class InNestedBSecond {
 	InNested inNested;
+	InNestedBFirst siblingReference;
 }