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

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.
Rename SOLVABLE_NAME_UPDATE_PREFIX to SOLVABLE_NAME_ADVISORY_PREFIX.
Add advisory object.
Add advisorylist object.
Add advisoryref object.
Add advisoryreflist object.
Add hy_package_get_advisories method.
api change: deprecate hy_package_get_update_*.
tests: add tests for advisory object.
tests: add tests for advisoryref object.
py: add _hawkey.AdvisoryRef type.
py: add _hawkey.Advisory type.
py: add _hawkey.Package.get_advisories method.
tests: py: add tests for hawkey.Advisory type.
tests: py: add tests for _hawkey.AdvisoryRef type.