Subsite does not prefix its navigation

[martymichal]

Description of the problem

Subsites added to a documentation do not prepend base_path to the path of the documentation files. This causes problems when there are documentation files with the same file structure.

How to reproduce

File structure

├── bar/
|   ├── docs/
|   |  ├── index.md
|   ├── mkdocs.yaml
├── docs/
|   ├── index.md
├── mkdocs.yaml  

Main documentation (Foo)

mkdocs.yaml file contains:

site_name: Foo
plugins:
  subsite:
    sites:
      - base_path: Bar
        nav_path: Subdocumentation/Bar
nav:
  - Index: 'index.md'
  - Subdocumentation:
      - Bar: []

Sub documentation (Bar)

site_name: Bar
nav:
  - Index: 'index.md'

Current behaviour

Navigating to Index shows the index page of documentation Foo.
Navigating to Subdocumentation/Bar/Index shows the index page of documentation Foo.

Desired behaviour

Navigating to Index should show the index page of documentation Foo.
Navigating to Subdocumentation/Bar/Index should show the index page of documentation Bar.