adjusts the way scripts are sourced to always use the full path from the root of the buildpack, so shellcheck can find the scripts without needing the shellcheck source=... directive
stops leaking some internal env vars to user-facing subprocesses
removes some duplicate sourcing of scripts and use of shopt
removes some low value historic code comments (that make the classic mistake of repeating what the code does, and not adding anything new, such as why)
This has been split out of a later PR to ease review.
This:
shellcheck source=...directiveshoptThis has been split out of a later PR to ease review.