don't sent uninstallable rc-only versions via EDSP

don't sent uninstallable rc-only versions via EDSP

Versions which are only available in dpkg/status aren't installable and
apt doesn't pick them as candidate for this reason – for the same reason
such packages shouldn't be sent to an external solver via EDSP. The
packages are pinned to -1, but if the solver has strict pinning disabled
it could end up picking this version anyhow – which is a request apt can
not satisfy.

Reported-By: Maximiliano Curia <maxy@debian.org> on IRC
(cherry picked from commit 33190fe3d3c200dcd417cd336f9db11f5f4408d5)