Support for generating qualified model package/namespace

The generator will currently place all generated model files in the same folder. This doesn't work well with larger models that may support different versions of the same API (requiring all model files to have unique names). The translation of schema names to model names also does a significant amount of character substitution, changing case etc. of the schema names, which further separates the generated code from the assigned schema name.

For example the following

 /my/path:
    post:
      ....
      requestBody:
        ...
        content:
          application/json:
            schema:
              $ref: '#/components/schemas/some.namespace.Model'

will generate a SomeNamespaceModel class file in the io.swagger.model namespace instead of a Model class in some.namespace

Because there's a significant amount of conversion of model names by legacy (transliterating _. etc to title case.
I propose to add a new flag to support generating fully-qualified names: useFqn

The issue identifies model-name translation and namespace generation as the relevant areas, but names no files, tests, or entry points. Start by locating those generator paths and determine how a proposed useFqn option would flow through them; done means qualified schema names can produce the intended namespace and model name without legacy substitutions.