Remove ebuilds with dead upstreams where possible

[Elizafox]

Repoman shows a lot of ebuilds have dead upstreams (their sites 404). If the upstreams truly are dead, and nothing depends on them, they should be removed from tree.

In addition, a sweep should be checked for the last bump of the ebuild. Those not bumped for more than 2-3 years should be evaluated.

[Elizafox]

@awilfox has started on this arduous task, but I'm not assigning it to them.

[mjeveritt]

Yeah I don't mind contributing to this one too, as it fits with the EAPI-0 project and tree-cleaning policy which I'll try to get started on based on what's already, and continues to be, discussed.

Perhaps we address this per-EAPI? ping @awilfox ?

[Elizafox]

It's better to start with repoman and see what returns a 404 and what doesn't, then work from there.

It may be useful to analyse what packages have not had commits since the git migration a year ago, also check what is m-n.