Use or remove "package" lines from votes

Currently dir-spec permits "package" lines in votes that can be used to vote on what the digest and location for a particular version of a software package is. This is not present in any votes I've seen.

We should either use this, or remove it from the spec (and any code that supports it in tor).

Roughly around here: https://spec.torproject.org/dir-spec#n1722