Relation to fpm-registry repository

[awvwgk]

While we already have a repository at fortran-lang/fpm-registry it seems best to start from scratch to build the registry backend. This means that the packages listed there have to be resubmitted once the registry backend goes online, but this should be a minor issue as there are only few registered packages and almost all maintainers are still active in the Fortran community.

We can however use the already registered packages in the other repository as example packages for testing our registry backend.