in preparation for provides support. implements also some
dependency satisfaction helper routines.

ref #574.

still todo:
  - 'fix' is missing
  - 'del -R' does not work
  - 'upgrade' does not do self-upgrade first

... and a lot of testing.

this allows quite some optimizations to running time and memory
requirements.

This applet can be used to generate data for graphviz tools.
Useful to visualize package dependencies, and possible errors
in the repository.

Usage examples:
  apk dot gnome-desktop | tred | dot -Tpng gnome-desktop.png
    -- to generate simplified dependency chart of gnome-desktop
  apk dot --errors | dot -Tpng index-errors.png
    -- to generate chart visualizing dependency errors in index