[xenial] Support branched Trusty/Xenial packaging build logic

[eloquence]

Debian packages should be stored in build/trusty or build/xenial, depending on build target. We need to clean up build targets and directory usage to avoid clobbering.

Branched build logic is necessary because dependencies for the securedrop-app-code package have changed; apache2 should be explicitly required; apache2-mpm-worker should be omitted.

Part of #3204.