RFE: Improve messaging when `rpmspec` is missing on an RPM-based distro

On OpenSUSE, rpmspec - i.e. the rpm-build package - is not installed by default. Because of this, commands like apkg build-dep fail with a pretty vague

Command not found: rpmspec -P <...>

from which it is not immediately obvious whether it's an issue with the package specification, or something is missing for apkg itself to work. I would suggest adding a message to lead the user to install rpm-build and tell them it's required for apkg to work correctly on an RPM-based distro.