Policy - lock down DiffusionSymbolQuery repo-loading code

Summary: Ref T7094.

Test Plan: couldn't really test this - how does one get symbols going nowadays given they are acanist project based?

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7094

Differential Revision: https://secure.phabricator.com/D11584