ansible.constants was calling expanduser (by way of shell_expand_path)
on the entire configured value for the library and *_plugins
configuration values, but these values have always been interpreted as
multiple directories separated by os.pathsep. Thus, if you supplied
multiple directories for one of these values, typically only the first
(at least on *nix) would have e.g. "~" expanded to HOME.
Now PluginLoader does expansion on each individual path in each of
these variables.
PluginLoader._get_paths, as of 391fb98e, was only finding plug-ins that
were in a subdirectory of one of the basedirs (i.e. in a category
directory). For example, action_plugins/foo.py would never be loaded,
but action_plugins/bar/foo.py would work.
This makes it so that "uncategorized" plug-ins in the top level of a
directory such as action_plugins will be loaded, though plug-ins in a
"category" subdirectory will still be preferred. For example,
action_plugins/bar/foo.py would be preferred over action_plugins/foo.py.
PluginLoader.add_directory() can receive None from, for example,
Inventory.add_directory(self.basedir()) if host_list is a custom list.
None has no reasonable interpretation other than ignore it.
