Add support for importing interfaces. (#3726)

Interface support is pretty skeletal so this may need additions later,
but I think it's still worthwhile to fill in the necessary bits now.
With this change, the expectation is then that everything we have right
now which _can_ be imported, is supported for import (at least for the
"current package, no overlap" case).

toolchain/check/handle_class.cpp

@@ -132,7 +132,7 @@ auto HandleClassDefinitionStart(Context& context,
   auto& class_info = context.classes().Get(class_id);
 
   // Track that this declaration is the definition.
-  if (class_info.definition_id.is_valid()) {
+  if (class_info.is_defined()) {
     CARBON_DIAGNOSTIC(ClassRedefinition, Error, "Redefinition of class {0}.",
                       SemIR::NameId);
     CARBON_DIAGNOSTIC(ClassPreviousDefinition, Note,