Pull default branch name automatically for github packages

[KronosTheLate]

Using GitHub.jl, one can automatically find the default branch for repositories. It should be possible to save a dictionary mapping a package name to a default branch to a (CSV?) file, and use that file to determine branchnames. If the key is missing for any package, it could be added as needed, which should be rare enough (only when adding new packages) that we would not require GitHub.jl to make authentuicated requests.

Also, potential homepages could be polled using GitHub.jl, which might be a nice link to have handy.

This would all just make it slightly easier to add new packages, at a one-time cost. But for now, there is more important and low-hanging fruit, and I think that this is not currently a priority.