Put code and indices in different branches

[alexarchambault]

Right now, both the code and the indices of this repo live in the same branch, master.

To avoid the temptation of modifying the indices right from user submitted PRs, maybe the indices should be moved to a branch different from the one of the code.

Maybe copying the current master branch to main (making it the default), and keeping the former master for indices, could work? Ideally, the indices' branch shouldn't change, as it's hardcoded in the coursier repo.

The only issue I see with this scheme is for Maven packages: if the tags go on main, we have to be sure master is in sync with main when pushing bits of it to Maven Central.