Rework package-installs to collapse on build host

Instead of doing the work in the image of parsing through the element's
package-install declarations, we can squash it on hostside, where we
have both YAML and JSON available to us, and then emit a single
pre-processed file into the target to be used later.

Change-Id: I3f182aa3aae0a79b2f3ea4e66c1878ad12878b0a