Currently, emerge can get very far into the dependency calculation before realizing that it has been given one or more arguments for packages that don't exist. It would be better to fail faster.
We want to detect the packages that don't exist in the depgraph._select_files method, around here:
> if "/" in x.split(":"):
> args.append(AtomArg(arg=x, atom=Atom(x, allow_repo=True),
> expanded_atoms = self._dep_expand(root_config, x)