Split validate package

[fredbi]

The validate package suports two related but different use-cases:

• validating swagger specs
• validating data against schemas

I would say that validating swagger specs is itself broken in:

• validating a swagger schema
• running extra rules

This mix of features makes the validate package difficult to document, maintain and guarantee against regression, especially the very sensitive data against schema part, which is used by go-swagger model generator.

In my opinion, we should split the package in two parts:

• a package dedicated to spec validation, and possibly json-schema validation (errors as go-openapi/validate/Result)
• a package dedicated to data validation against json schema (errors from go-openapi/errors)

We could achieve that without breaking the existing interfaces using Go 1.9 type aliasing (https://github.com/golang/proposal/blob/master/design/18130-type-alias.md). However, that would break our current commitment for compatibility with 1.8.