use pathlib for file operations across the board

use pathlib for file operations across the board

The command, config and script modules now rely on ``pathlib.Path`` for
internal path manipulations, instead of ``os.path()`` operations.   Public
API functions that accept string directories and filenames continue to do
so but also accept ``os.PathLike`` objects.  Public API functions and
accessors that return paths as strings continue to do so.   Private API
functions and accessors, i.e. all those that are prefixed with an
underscore, may now return a Path object rather than a string to indicate
file paths.

Change-Id: I7c79cdecd7e866c7086422079aa92077816acec3