[bug] Tox picking up a `toxfile.py` is untraceable: plz show it in `--version` output

[webknjaz]

toxfile.py allows declaring plugin-compatible hooks, documented at @ https://tox.wiki/en/latest/plugins.html. And it's a local in-project plugin in that sense.

However, running tox --version does not show it anywhere and also, tox_append_version_info() is one thing declared on the plugins page does not work.

I think it would be expected to display it in the registered plugins: list or as a separate entry. Additionally, showing what tox_append_version_info() returns would be a useful way for people to give hints for the local automation setup, and it'd be in line with the generic feel of it being a plugin.