Dpkg::Deps: tweak the comparison algorithm to sort > lower than <

Dpkg::Deps: tweak the comparison algorithm to sort > lower than <

* scripts/Dpkg/Deps.pm: Add a compare function that compares
dependencies more intelligently than a comparison on their
string representation. In particular we want >= and >> to sort
lower than << and <= so that intervals are nicely displayed
as "a (>= 1), a (<< 2)" instead of the ugly "a (<< 2), a (>= 1)".