Handle named default exports separately

Ticket: #25857 (closed)

This MR changes the way we check for duplicate exports of named default declarations. They are now treated entirely separately from other exports, because in an export list of the form

module M ( default Cls, Cls )

the default declaration does not export the class Cls, but only its default declarations.

This MR also fixes a bug in Backpack wherein named default exports were getting dropped entirely. No test for that.