Project.pm: slightly relax does_exist check
In the case where forks have been kept but the forkee itself has actually
been removed, the valid_name check will fail since it requires the forkee
to still exist.
Unfortunately this also causes the does_exist check to fail on these forks
even though they actually do exist (and load will succeed).
Relax the does_exist check so that if the forkee does not exist it's
enough that the fork directory does exist before actually checking for the
existence of the project itself.