v0.0.19 Release

This release most notably adds fields to the Package structure for greater support of package manager features.
Various fixes and additions are included as well.

Documentation for this release can be found here: https://quay.github.io/claircore/v0.0.19/

#155 update photon security url to new location
#153 Add architecture property to Package
#152 Must Parse Digest
#151 migrations: improve extension error reporting
#148 Add module property to Package
#147 Normalized Severity Fix