process_manifests needs a public system label

[alice-i-cecile]

I need to configure the ordering in which two dependent manifests are loaded. While I could do this with system ordering, bevyengine/bevy#13064 means that I need a true system label to do so nicely.

The existing system label is inadequate: we need access to the full type. We can probably add a marker generic to ProcessManifestSet though, and use () as a default which means "all systems".