Try to install non-KDE build dependencies automatically

See https://bugs.kde.org/show_bug.cgi?id=402428

As title indicates, kdesrc-build should do what it can to automatically detect which non-KDE dependencies are missing and get those installed as well.

This is a critical enabler to making it easier to onboard new contributors. There's a lot of commentary on the bug itself but I'd like to keep the narrative running here.