Consider splitting packages.py into smaller modules

[jherland]

This grew out or the discussion at #326 (comment):

@mknorps writes:

Just to let you know, I had a bit of hesitation if this function should be in this module, but I convinced myself that this is the only reasonable place. packages could be otherwise called extract_mapping, though it is not exactly the same that is its function.

@jherland writes:

Yeah, I also see that packages.py has gradually grown to a size where it could maybe benefit from being split up into several modules. Maybe that split would result in completing the trinity of extractors? wink I haven't looked at it closely enough yet to form an opinion on what exactly the split would look like...