When available, pass path, line and repository hints to external symbol queries

Summary:
Depends on D18936. Ref T13047. Third parties can define external symbol sources that let users jump to PHP or Python documentation or query some server.

Give these queries more information so they can try to get better results: the path and line where the symbol appeared, and any known repository scope.

Test Plan: Wrote a fake external source that used this data, command-clicked a symbol in Differential, saw a fake external symbol result.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13047

Differential Revision: https://secure.phabricator.com/D18937