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