If a new pseudo-essential package was added to testing *and* it is
mutually-exclusive with an existing member, britney would incorrectly
flag it as uninstallable (unless another change triggered a cache
invalidation of the pseudo-essential set).
With this commit, the cache invalidation is now done based on whether
we add something that *might* be in the (effective) pseudo-essential
set. It is an overapproximation as there are cases where the
invalidation is unnecessary, but at the moment it is not a performance
concern.
Closes: https://bugs.debian.org/944190
Signed-off-by: Niels Thykier <niels@thykier.net>