Changes for week of Jan 20

- Misc typos/clean-up
- Require `target` to NOT point to a model entity that uses `ximport`.
  Meaning, `target` is not transitive - to keep things simple.
- Tweak usage of `target`. Can be on any relative URL/URI, and if value starts
  with `/` then it must match the xid template. Similar for `relation` but
  `relation` must be an xid... but `target` is optional for all types.
- Add "deprecated" to messages and schemas
- define "xref" as a "relation" type with "target"="self-xid"
- add "modelversion" as a model label

Fixes: xregistry#236
Fixes: xregistry#226
Fixes: xregistry#227

Signed-off-by: Doug Davis <[email protected]>