pkg: Add limited --debug/-d support

Add an internal debug level global:

• Level 1 (-d) currently does nothing.
• Level 2 (-d -d) enables libfetch debugging (quite verbose) so it's possible to see what pkg is attempting to download without having to sniff traffic.

Reviewed by: debdrup, bapt
Differential Revision: https://reviews.freebsd.org/D35756