[spec-model] Cache content of Swagger objects with identical paths

spec-model can create multiple instances of `Swagger` objects pointing to the same path, if the same path is referenced mutliple times (e.g. in multiple tags, $ref from multiple other swagger, etc).

This should be fine for correctness, but we could improve perf by caching these objects, to avoid reading and parsing the same content multiple times.

The gap is Swagger objects point to their Tag. So we might need to refactor into a SwaggerContent object for the contents (which are cached by path), and Swagger includes the tag.