Move the fallback on providers from backend to frontend.

This reverts commit e28973169d2e5eda8b64ebdda11ece0dc761d978.
This code might fit better in the frontend than in the backend finally.
Ref: http://www.archlinux.org/pipermail/pacman-dev/2007-November/010150.html

I also changed it for fixing FS#8763 :
if there is exactly one provider, pacman will pull it and print a warning.
if there are several providers, pacman will list them and fail. It's up to
the user to pick one. Add sync501 pactest to reflect that.

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