extend or create new module to check for redundant versions of a package

[prometheanfire]

cleanup seems to do some of the work, but I don't think it checks to see if a particular version is needed by other packages.

The aim is to get the least (and newest) amount of package versions for a package so the others can be removed.

[radhermit]

Under the current architecture, this would require keeping a revdeps cache (https://github.com/pkgcore/pkgcheck/issues/285) updated in order to be usable at all.