backtracking and logging output

[rob...@robertcollins.net]

Hi, during collection we're quite chatty: 'collecting $name', 'requirement already satisfied' etc etc.

With a backtracking resolver this becomes hugely verbose.

I'm seeking input on this. My preferred approach is to:

 - push the incremental activity down to debug

 - if we find a solution log at a user visible level the actions we're going to take

We'll still show a progress bar etc when we download candidates, so the user will still see activity.

Thoughts?

-Rob