sack: Also look in /usr/share/rpm for Packages

Followup on https://github.com/akozumpl/hawkey/pull/47

In the rpm-ostree model, the RPM database lives in /usr/share/rpm, and
is immutable.  Let's look there first, and then fall back to the old
mutable database.

This removes the #define from the public API, as it's not useful to
external consumers.