Repoman Manifests: Move some additional code to manifests.py

Move the digest check to the Manifests class.
Optimize usage to use the new qatracker system.
Repoman metadata.xml: Create a new PkgMetadata class

Move the metadata.xml checks to its own class in checks/ebuilds/pkgmetadata.py.
Move the herd_base determination to checks/herds/herdbase.py in get_herd_base().