parser: add a validation mode
mkaurball.in: chmod 644 since this file should never be run
parse pacman DB directly instead of pacman -Si

This lets us cast a wider net for testing attributes
compare: objectify differences
mkaurball: reverse logic to determine copy v. generate

The logic was the wrong way around, meaning that if srcinfo_path was not
set (default), the script tried to copy it into the tarball directory,
causing the script to fail. If it was set, then the script created a new
.AURINFO anyway.