Plug-ins loaded from top-level plug-in directory

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.