Try and prevent people from adding github archive urls.

If you use the archive URL directly, you end up having non unique
distfiles named 0.9.5.tar.gz or v2.2.2.tar.gz, and soon, another port
ends up having the same version, and boom.

Reviewed by: bdrewery
Sponsored by: Absolight
Differential Revision: https://reviews.freebsd.org/D15039