[descriptors] Move DeclarationDescriptor#findPackage helper to core

There is nothing serialization-specific about this function

core/descriptors/src/org/jetbrains/kotlin/descriptors/descriptorUtil.kt

@@ -105,4 +105,9 @@ fun FunctionDescriptor.isTypedEqualsInValueClass(): Boolean {
 
 fun FunctionDescriptor.overridesEqualsFromAny(): Boolean = name == OperatorNameConventions.EQUALS
         && valueParameters.size == 1 && valueParameters[0].type.isNullableAny()
-        && contextReceiverParameters.isEmpty() && extensionReceiverParameter == null
+        && contextReceiverParameters.isEmpty() && extensionReceiverParameter == null
+
+tailrec fun DeclarationDescriptor.findPackage(): PackageFragmentDescriptor {
+    return if (this is PackageFragmentDescriptor) this
+    else this.containingDeclaration!!.findPackage()
+}