pkgctl repo configure: Sanity check if we are in a packaging repo

Currently we just configure whatever, so when doing a pkgctl repo clone --universe in the wrong folder I accidentially configured my linux source tree with pkgctl repo configure.

We should:

• check if we are configuring a packaging repo (is it gitlab.archlinux.org and has something with "packaging" in the remote URL)
• not invoke configure for existing repositories