Add a linter rule for unnecessary symbol aliases

Summary: Add a linter rule to detect symbols which are aliased with the same name, such as use X\Y\Z as Z. This is unnecessary and is more-simply expressed as use X\Y\Z.

Test Plan: Added unit tests.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D14557