Better support with `macros` plugin to override `extras`

I'm not sure if I should ask this here, or at the `macros` repository.

Consider the following setup:
```yaml
# /mkdocs.yml
site_name: Cats API

nav:
- Intro: 'index.md'
- Authentication: 'authentication.md'
- API:
- v1: '!include ./v1/mkdocs.yml'
- v2: '!include ./v2/mkdocs.yml'

plugins:
- monorepo

extras:
version: v2 # default
```

```yaml
# /src/v1/mkdocs.yml
site_name: versions/v1

nav:
- Reference: "reference.md"
- Changelog: "changelog.md"

extras:
version: v1
```

```yaml
# /src/v2/mkdocs.yml
site_name: versions/v2

nav:
- Migrating to v2: "migrating.md"
- Reference: "reference.md"
- Changelog: "changelog.md"
```

I would love to have the version variable be overridden by the v1 in the earlier version, but it looks like the included extras are not evaluated.

Start with the root mkdocs.yml and the included src/v1/mkdocs.yml and src/v2/mkdocs.yml to understand how the monorepo plugin processes included configuration and extras. Verify the desired behavior by checking that v1 resolves version as v1 while the root and v2 retain the default v2 value.

Written by the indexing model from the issue text.