Provide public API with path to project for plugins

[Czaki]

What's the problem this feature will solve?

After discussion in #2886 I have decided to create a plugin to simplify minimum requirements testing https://pypi.org/project/tox-min-req/. But during development, I meet problems with determining the path to the project. Finally, I decide to use tox_env.core._root
https://github.com/Czaki/tox-min-req/blob/1e32144066ef8f41aae15abb7c94b639357f8be5/tox_min_req/_tox_plugin.py#L45

I also observe that arguments contains a string which part is the path to the tox file. But I do not find documentation about this.

Describe the solution you'd like

Provide public API to determine the path to the project or update documentation of tox_on_install or ToxEnv with a point to the proper property.

Alternative Solutions

Maintenance on the side of the plugin.

Additional context

[gaborbernat]

tox.core['package_root'] is the path ahead per https://tox.wiki/en/latest/config.html#package_root (the tox core config keys are keys to the core object)