Wrong $ref for path in nested dir processing in v1

[ZsergUA]

In case $ref for path is pointing to a file in nested directory, internally defined refs are using wrong base directory.

While having swagger.yaml with:

paths:
  /some_path:
    $ref: './subdir/some.yaml'

and some.yaml with:

get:
  responses:
    200:
      schema:
        type: object
        properties:
          some_property:
            $ref: './some_definitions.yaml#/SomeProperty'

will try to resolve some_definitions.yaml in the swagger.yaml root directory instead of subdir

[ZsergUA]

pull request with fix #1224

[gracekarina]

thanks! @ZsergUA
the PR is now merged.