don't remove outdated binaries if they are needed for dependencies

[Martin Pool]

(I'm not totally sure this is valid; if not, reject it.)

Suppose we have foo-1.1 and foo-dep-1.1, which needs exactly foo=1.1.

Then a new 1.2 release of both of them comes out, and someone uploads foo-1.2, but for some reason there is a delay befare foo-dep-1.2 is uploaded.

I think the PPA will remove foo-1.1, leaving people a bit stuck.