Allow x-swagger-router-controller value to be a directory of operation files

[TuckerCowie]

Given a swagger config:

  "paths": {
    "/pets/{id}": {
      "get": {
        "operationId": "getPetsById",
      }
    }
  }

and a router config:

{
  swaggerRouterOptions: {
    controllers: './controllers'
  }
}

and the following directory structure:

controllers/
`-- Pets/
    `-- getPetById.js

This request will invoke getPetById as a function

GET /api/pets/123
200

Otherwise, all preexisting functionality is present.

[JonClayton]

This is pretty cool since it allows easier separation of the individual endpoint handling functions without an artificial and long top level controller file

[whitlockjc]

So you want to break down the controller into separate, per-handler files?

[TuckerCowie]

Correct. In a microservice architecture or in servers that only have one top level business concept, certain routes may implement some middleware and utilities while others may not. In our use case sometimes we are dealing with CRUDing basic JSON resources, while other times we are exposing business logic through an endpoint. Splitting the controlling logic into per-handler files helps alleviate unnecessary overhead for those routes that do not share any implementation details even though they may share a common parent route.