in `xxx` allow `type` as a synonym for `xxx_type`

Plans become much easier to read if we allow people to simply refer to a `type` in their artifact/service/requirement definitions.

Calling them `aritfact_type` and `service_type` and `requirement_type` is just long-winded and redundant in almost all cases.

I suggest accepting the longer form `xxx_type` for an `xxx` in case there is any ambiguity.