Support /usr/share/apt/verify.d

See https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1060181

I think we should use a path like /usr/share/apt-verify/verify.d/ instead, to not hijack the unused /usr/share/apt/ hierarchy. We could use /usr/share/apt-verify.d/ directly but that is a bit inflexible if we ever need to create more share-stuff.