add rpmdb support; enhance integration tests

integration/test-fixtures/image-pkg-coverage/var/lib/rpm/generate-fixture.sh

@@ -0,0 +1,23 @@
+#!/usr/bin/env bash
+set -eux
+
+docker create --name generate-rpmdb-fixture centos:latest sh -c 'tail -f /dev/null'
+
+function cleanup {
+  docker kill generate-rpmdb-fixture
+  docker rm generate-rpmdb-fixture
+}
+trap cleanup EXIT
+
+docker start generate-rpmdb-fixture
+docker exec -i --tty=false generate-rpmdb-fixture bash <<-EOF
+  mkdir -p /scratch
+  cd /scratch
+  rpm --initdb --dbpath /scratch
+  curl -sSLO https://github.com/wagoodman/dive/releases/download/v0.9.2/dive_0.9.2_linux_amd64.rpm
+  rpm --dbpath /scratch -ivh dive_0.9.2_linux_amd64.rpm
+  rm dive_0.9.2_linux_amd64.rpm
+  rpm --dbpath /scratch -qa
+EOF
+
+docker cp generate-rpmdb-fixture:/scratch/Packages .