repositories: make sure repositories not only differ in casing

Repositories only differing in case cause problems:
* it can't be stored on case insensitive filesystems (Windows and MacOS)
* some databases can't easily handle case sensitive queries
* users will most certainly be confused by names that only differ in case

We will keep trying to be case sensitive on systems that can ... but on some
systems wrong casings might work. We don't care.

The validators are changed to prevent mixed case repo and repo group names.

Repository sensitivity tests are removed, and insensitivity tests are added
instead.