Framework for external symbol search

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