In Diffusion, split service URI resolution from conduit client construction

Summary:
Ref T7019. Ref T7034. In both proxying cases, we want to proxy the request but can not do so over Conduit.

Split the URI resolution apart from Conduit client construction so we can just pull an SSH or HTTP/S URI out of the repository without getting an entire Conduit client.

Test Plan: Browsed around a service-hosted repository. This diff has no behavioral changes.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7034, T7019

Differential Revision: https://secure.phabricator.com/D11491