Store the repository root in repo config files.

Hence we do not relay on the global state anymore (as this was previously computed to be <current-opam-root>/repo/<name> which didn't work well when using the client API).
Tentative fix for bad error message on 'opam update' when a repo is not available

So no normally, no more `OpamParallel.Error(_,1)`
Do not check for updates for HTTP dev package (which are not really dev packages anyway)