Let the user know 'apt-file' is needed when appropriate

As a new user of jhbuild it took me a bit of time to understand that the sysdeps command, when used on a Debian-based system, needs apt-file to be installed before it could figure out which packages to install. I eventually figured it out with a bit of searching (and landed on this issue, but I think we could be a little more helpful to the user when we log that there is nothing available to install.

Before	After