Support type-registrations in seeAlso files

[jeff-zucker]

As brought up repeatedly by members of the interop team, having all type registrations in the same document means that any app can see the registrations of all other apps. I believe that there are many apps that don't care about that or actively want to be visible, but obviously, there are some that do care.

If we permit type registration statements in seeAlso files (possibly linked to from the public or private type-index?), we can accomodate apps that care. The seeAlso file could be permissioned for that app alone meaning that no other app could see its registrations but the app itself would be able to see them. If the seeAlso file is randomly named, that would mean that no other app would be aware that the privacy-wanting app was being used, or where it stored its data, or what types it used. We could also have a solid:restrictedTypeIndex predicate that took multiple objects rather than using rdfs:seeAlso.