The old pattern of using a separate 'OCI' flag was very ugly
internally in the code once it was extended to flatpak bundles and
flatpakrefs - using a different URI scheme means that the nature
of the remote can't be accidentally lost in some part of the code.
Probing would be possible as well, but would make it difficult to
add a remote when offline, and also doesn't deal well with the
fact that our data layout is different for the two types of remotes -
the type of remote could change at any point!
As a side effect this change enables flatpakrefs and flatpak bundles for OCI
registries.
d7d05a8619