[keyserver] Update URL facts function to use dynamic loading mechanism

Summary:
The previous diff introduced this mechanism, but did not actually update callsites.

The overall goal is to make these files optional. The next diff will update the types to be optional.

Depends on D3974

Test Plan: Make sure I can still use the keyserver

Reviewers: atul, palys-swm

Reviewed By: palys-swm

Subscribers: Adrian, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3975