Validate entrypoint group name

[FFY00]

Per https://packaging.python.org/en/latest/specifications/entry-points/#data-model, the name of entrypoint groups must match ^\w+(\.\w+)*$.

[henryiii]

This is a recommendation, not a requirement, though.

(For the name, that is; for the group, yes)

Edit: "name of the entrypoint groups", I guess I can't read. Yes, you are correct. :)