We don't currently want the minimal-plus target to be user-facing. It's
intended only for other Fedora variants.
Let's support "hidden" manifests by having their names end in
`.hidden.yaml`. Then, make `minimal-plus` hidden.
I considered instead having them start with `.` to match
well-established semantics, but I don't like either that this hides them
from an `ls` in the git repo (or even with `-a`, puts them higher up
whereas I want it to be close to `minimal.yaml`).
I also considered leveraging the existing symlink hiding semantics, but
it also felt awkward to rename a file and add a symlink to it just for
the purpose of hiding it.
