Move cmdlib.py to cmdlib/__init__.py

cmdlib.py has grown *really* too much. Move it into its own package to
allow splitting it further.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Signed-off-by: Thomas Thrainer <thomasth@google.com>
Reviewed-by: Klaus Aehlig <aehlig@google.com>