Support by-hash files for verbatim publications

[daviddavis]

If using a verbatim publication and the upstream has by-hash files, publish them along with the other repo metadata files.

I think that this feature should probably ignore the APT_BY_HASH setting and rather just mirror whatever upstream is doing since upstream will have Acquire-by-Hash setting in its Release file and pulp_deb just mirrors this setting from upstream.

This feature is probably related to this other feature I've filed to sync using by-hash files: #1031.