Find a better way to define build dependencies between packages

[pzakha]

Right now we have some build dependencies between a few packages: bcc & bpftrace, and java8 and the SSO app (to be coming soon).

The way we currently handle this is by installing the package that we have dependencies on after building it, or triggering a build for that package if we haven't built it yet. The code construct is a bit awkward, for instance for bpftrace we have:

	if ! dpkg-query --show libbcc >/dev/null 2>&1; then
		echo_bold "libbcc not installed. Building package 'bcc' first."
		logmust "$TOP/buildpkg.sh" bcc
	fi

We should be able to codify those dependencies in better ways.