Fix a Calendar import issue where we looked up attendees by object instead of name

Summary:
Ref T11801. This issue led to the stack trace in T11801#199042.

It wasn't obvious that this was wrong because the recover-on-duplicate-key code made it work correctly.

Test Plan: Imported an event with external attendees with no warnings in the log.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11801

Differential Revision: https://secure.phabricator.com/D16804