filter old packages coming from rmadison

So that cme can better detect out of date versioned dependencies.

Up to now, a version listed by rmadison was considered as recent enough.

But look at this output of rmadison
$ rmadison libpath-tiny-perl
libpath-tiny-perl | 0.058-1       | oldoldoldstable | source, all
libpath-tiny-perl | 0.100-1       | oldoldstable    | source, all
libpath-tiny-perl | 0.108-1       | oldstable       | source, all
libpath-tiny-perl | 0.118-1       | stable          | source, all
libpath-tiny-perl | 0.118-1       | testing         | source, all
libpath-tiny-perl | 0.118-1       | unstable        | source, all

rmadison keeps the history for too long time to be practical.

Since we do not support backports on release older than oldstable, the
oldold (or more) release are pruned.

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