[packagekit] Python dependencies

[Tom Parker]

I've just committed some autoconf magic to check that the python apt
extensions have been installed if someone wants to use the apt
backend, 'cause otherwise they won't work. Other backend authors
probably want to do the same for their backend's python dependencies,
but I've noticed that in general (holds for both apt and yum backends
at least) if the dependant packages aren't on a system (and the python
apt extensions are not installed by default on Debian systems at
least), then they will crash out immediately.

Thoughts:
1) The autoconf magic mentioned above + package management for people
not doing the builds themselves should solve *most* of this.
2) Should the backend spawn parser have checkers to notice this and to
report stuff back to the users if everything fucks up? Is it better to
give the user a python stacktrace (or at least a text file to give to
developers similar to what nautilus does when it crashes) rather than
just saying "random backend error"?

Tom