Look through extend require in an interface or named constraint in name lookup (#6630)

Add the required facet type as an extended scope of the containing
interface/named constraint, and teach name lookup to look for extended
scopes in named constraints.

This makes name lookup work properly when the facet type does not have a
specific that involves `Self`. Support for `Self` needs further work in
another PR.

Note that when an _interface_ requires another interface, this PR lets
us find the name, but we still fail to find a witness for the interface
named through `extend require`, and this is future work. For a named
constraint, things work correctly as the identified facet type chases
through the named constraint and includes the required interface, so
impl lookup is able to provide a witness.

toolchain/check/import_ref.cpp

@@ -2779,9 +2779,6 @@ static auto AddInterfaceDefinition(ImportContext& context,
   new_interface.body_block_id =
       context.local_context().inst_block_stack().Pop();
   new_interface.self_param_id = self_param_id;
-
-  CARBON_CHECK(import_scope.extended_scopes().empty(),
-               "Interfaces don't currently have extended scopes to support.");
 }
 
 static auto TryResolveTypedInst(ImportRefResolver& resolver,
@@ -2930,10 +2927,6 @@ static auto AddNamedConstraintDefinition(
       context.local_context().inst_block_stack().Pop();
   new_named_constraint.self_param_id = self_param_id;
   new_named_constraint.complete = import_named_constraint.complete;
-
-  CARBON_CHECK(
-      import_scope.extended_scopes().empty(),
-      "Named constraints don't currently have extended scopes to support.");
 }
 
 static auto TryResolveTypedInst(ImportRefResolver& resolver,