manual: restructure 'Adding packages' chapter

The depends-on-vs-select part of the manual really deserves its own
section title (especially because it is referred to and the xref gets
a 'sinpara' in PDF if the section doesn't have a title). So restructure
the surrounding sections to reduce the section nesting depth.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Acked-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>