allow to only search latest source package version

[josch]

Debian Sid often contains the same source package in multiple versions. This is the distribution of number of versions per source package right now, first column the number of source package and the second column how many versions these source packages have:

20940: 1
550: 2
31: 3
4: 4
4: 5
2: 7
1: 16

(the source package with 16 different versions is gcc-4.9)

It would be nice if one could limit the results to the latest source package version only. This would be very useful for the 592 source packages that have more than one version in sid.