inventory/__init__.py: further revision of implicit localhost

Combine all of the lists into a single localhost_list do cut down on duplication.
Rename add_localhost_to_hosts_cache_part_of_subset to _add_implicit_localhost_if_localhost_in_subset reflecting that it is a private method and that we add localhost to the inventory the same way that get_host eventually does via _get_hosts and __get_hosts.