properly parse comments in apt_preferences and deb822-style sources

apt_preferences and deb822-style sources used the specialized class
pkgUserTagSection to deal with comments before/after a given stanza, but
it couldn't deal with comments in the stanza at all.

codesearch suggests that nobody else does and a vastely superior way of
working with potentially commented files is implemented now, so we can
officially discourage the use of the old incomplete hack class.