Do not render declarations from other modules in diagnostic tests

Because PackageViewDescriptor may consist of several package fragments from
different modules (see LazyPackageViewDescriptorImpl#fragments), we now filter
out fragments from irrelevant modules before rendering them into the .txt

compiler/testData/diagnostics/tests/multimodule/duplicateClass/sameClassNameDifferentPackages.txt

@@ -33,12 +33,4 @@ public final class A {
 }
 
 package p {
-
-    public final class A {
-        // -- Module: <m1> --
-    }
-
-    public final class B {
-        // -- Module: <m1> --
-    }
 }