Error message gives wrong advice for allowed module names

Under some conditions, Puppet will issue an error message like Error: Invalid module name .gitignore; module names must be alphanumeric (plus '-'), not '.gitignore'.

That's wrong! You should definitely never have hyphens in your module names, but underscores are encouraged.

The docs do not list hyphens as valid characters in module and class names, and we've publicly stated for years that it's a bug that it works and we'll make it stop working at our earliest convenience.

Here's the relevant code:

https://github.com/puppetlabs/puppet/blob/master/lib/puppet/module.rb#L343