Make Validator::complete required

[dmontagu]

Removing the need for complete() is hard, it might even be impossible, definitely not with the hassle now.

Making complete required is very simple and probably worthwhile, feel free to do it here or create an issue.

Originally posted by @samuelcolvin in #494 (review)

The reason to do this is that it's easy to add a new validator type in pydantic core and forget that you need to implement this method if it references other validators (otherwise your DefinitionRefValidator-produced loc's will be wrong)

[aminalaee]

Since #525 is merged, this can be closed.

[samuelcolvin]

thanks so much.