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/packagePrivate.txt

@@ -11,7 +11,5 @@ package p {
 package
 
 package p {
-    private val a: kotlin.Int = 1
-    public val b: kotlin.Int = 1
     public val c: kotlin.Int = 1
 }