Make checking whether AUR packages exist fast

[gsingh93]

It's easy to verify that packages in the official repos exist, as you can simply see if the packages exist in the output of pacman -Ss. However, I don't know of any easy way to get the list of packages from the AUR, so instead the output of yaourt -Ss package_name needs to be checked for each AUR package, which is slow.

AurJson might be one way to get this package list.

Once this is fixed, the -s flag for skipping this verification should be removed.