(PUP-1676) Allow main as a directory environment.

Looking again at the settings code, the static sections in puppet.conf
currently shadow a directory environment of the same name, because we
check the configuration file first.  But I'm not seeing any reason to
specifically forbid main as a directory environment name.  Once static
environments are removed, we will need to address how to obtain
puppet.conf [main] settings without shadowing directory environments.