Provide tuning on strictness of internal parser

Allow iter_paragraphs to be tuned as to whether it splits paragraphs the separator is a line containing whitespace. When working with control, iter_paragraphs must split on whitespace-only lines #715558 for example). When working with Packages or Sources, does not split on whitespace containing lines and so neither should internal parser.

(Closes: #913274)

This merge sits on top of the previous one about warnings coming from not using apt_pkg but can be rebased off it if needed.