Support $ref in PathItem

[cebe]

https://github.com/OAI/OpenAPI-Specification/blob/3.0.2/versions/3.0.2.md#pathItemObject

| $ref | string | Allows for an external definition of this path item. The referenced structure MUST be in the format of a Path Item Object. If there are conflicts between the referenced definition and this Path Item's definition, the behavior is undefined.

This property is currently not recognized.

[cebe]

Add a test based on the example files in OAI/OpenAPI-Specification#1961 (comment)

this currently fails resolving.

[cebe]

Mystery of unclear semantics is explained here: OAI/OpenAPI-Specification#1038