Inline with recent packaging refactoring, need to consolidate the deb/templates/build_debfiles.py module with the deb/lib.py module. It's also doing redundant caching logic that should be removed.

This does part of it

https://git.opendaylight.org/gerrit/#/c/63360/

Imported from: https://trello.com/c/qG700SZ2/606-move-builddebfiles-logic-into-lib-clean-up-caching-its-doing#