Summary:
Ref T7984. With this, an install can add an ExternalSymbolsSource to src/extensions, which will include whatever
source they have.
Test Plan: search for php and python builtins.
Reviewers: joshuaspence, epriestley, #blessed_reviewers
Reviewed By: epriestley, #blessed_reviewers
Subscribers: Korvin, epriestley
Maniphest Tasks: T7984
Differential Revision: https://secure.phabricator.com/D13036
