Support releases without dune-project files

[samoht]

It's convenient to add a dune-project file at the root a non-dune project to ease release (good example: https://github.com/mirage/ocaml-solo5) - it'd be nice if the project name could also just be given on the command-line (and in that case, to not complain if a dune-project file is not present).

[NathanReb]

Indeed! We'll remove the need for a dune-project file in 2.0.0 as it is only used to determine the tarball name. We'll use the name of the repository in dev-repo for this as the repo ultimately is what the tarball contains.