A slightly more meaningful number than number/percentage of dependencies
using the given dependency is how many repos are dependent on it.

This provides a more actionable signal that the dependency is important
if a high number of repositories in the organisation use it, as well as
allowing a better indication when using several ecosystems, some of
which use fewer dependencies, and some using 5-10x more, which reduces
the impact that a given dependency could show as.

As this is a bit more of an interesting signal, we can use this for the
sorting, too.

Closes #493.