Update borg_github module to only allow projects names with underscores

[klonos]

There was a recent issue with https://github.com/backdrop-contrib/eu_cookie_compliance where the project name recently changed from eu-cookie-compliance to eu_cookie_compliance. This has caused the creation of two project nodes on b.org, and we are now faced with many problems related to how people can go about upgrading to recent versions of the project without losing their config.

While the specific use case of the eu_cookie_compliance module should be addressed separately, this issue here is to address the general problem at its root, by stopping projects created on GitHub with dashes in their names from creating respective project and release nodes on b.org.

[ghost]

Whats the problem with dashes? Do they cause issues other than duplication when a project changes to using underscores instead?