Comment 1 for bug 1680574

$ ubuntu-image -w /tmp/wd pc-amd64.assertion
$ ls /tmp/wd/root/system-data/var/lib/snapd/snaps/
core_1636.snap pc-kernel_60.snap

So that seems like the information you're asking for. I guess a file listing of that directory would be the manifest you want, right? Should we just add a -m/--manifest argument which names a file into which we would write these snap file names?