[IR] Add a method to get ModuleDescriptor from IrPackageFragment

Ideally, instead of this method, there should be a link
to IrModuleFragment. Unfortunately, it would require to big refactoring,
as some of IrPackageFragment implementations doesn't have any
IrModuleFragment inside, and are not located inside any
IrModuleFragment.

So for now, we just implement and use everywhere a single way of
getting the module descriptor, which respects a IrModuleFragment
link if it exists, and fallbacks to descriptor-based method
if it doesn't.

^KT-62623

compiler/ir/ir.tree/tree-generator/src/org/jetbrains/kotlin/ir/generator/IrTree.kt

@@ -618,6 +618,14 @@ object IrTree : AbstractTreeBuilder() {
 
         +symbol(packageFragmentSymbolType)
         +field("packageFragmentDescriptor", type(Packages.descriptors, "PackageFragmentDescriptor"), mutable = false)
+        +field("moduleDescriptor", type(Packages.descriptors, "ModuleDescriptor"), mutable = false) {
+            kdoc = """
+            This should be a link to [IrModuleFragment] instead. 
+               
+            Unfortunately, some package fragments (e.g. some synthetic ones and [IrExternalPackageFragment])
+            are not located in any IR module, but still have a module descriptor. 
+            """.trimIndent()
+        }
         +field("packageFqName", type<FqName>())
         +field("fqName", type<FqName>()) {
             baseGetter = code("packageFqName")