* Do not report export collision errors until the very end, because it
   is possible, during an update, for a new export to be added before an
   old one is semantically analyzed to be deleted. In such a case there
   should be no compile error.
   - Likewise we defer emitting exports until the end when we know for
     sure what will happen.
 * Sema: Fix not adding a Decl dependency on imported files.
 * Sema: Properly add Decl dependencies for all identifier and namespace
   lookups.
 * After semantic analysis for a Decl, if it is still marked as
   `in_progress`, change it to `dependency_failure` because if the Decl
   itself failed, it would have already been changed during the call to
   add the compile error.