Accept newline-delimited packages

[imjasonh]

with:
  packages: |
    git
    apko
    melange
  run: |
    melange version

This might already work today with enough YAML-fu, but we should at least test and document it if it is. For bonus points, ignore comments:

with:
  packages: |
    git # because of blah
    # TODO: remove these
    apko
    melange

(This might also already work, but should be tested/documented)