warning log about netifaces on import is annoying

Importing pypacker when netifaces is not installed results in a warning log. This is a little bit annoying, and I can't find a way to suppress it.

If it raised a warning instead of logging, it would be possible to suppress at least, but it still wouldn't be convenient.

I would suggest logging (or raising) a warning only when the relevant utils which are dependent on it are called.