Make bin/lipsum generate hanldle generator keys and arguments more clearly
Ref T12319. Currently, bin/lipsum uses substring matches against human-readable text to chose which objects to generate.
- Use separate selector keys which are guaranteed to be unique.
- When a match is exact, select only that generator.
- When a match is ambiguous, fail and warn the user.
Test Plan: Generated several types of objects, tried to generate ambiguous objects like "e".
Reviewed By: chad
Maniphest Tasks: T12319
Differential Revision: https://secure.phabricator.com/D17420