just-in-time creation for (explicit) negative deps

just-in-time creation for (explicit) negative deps

Now that we deal with provides in a more dynamic fashion the last
remaining problem is explicit dependencies like 'Conflicts: foo' which
have to apply to all architectures, but creating them all at the same
time requires us to know all architectures ending up in the cache which
isn't needed to be the same set as all foreign architectures.

The effect is visible already now through as this prevents the creation
of a bunch of virtual packages for arch:all packages and as such also
many dependencies, just not very visible if you don't look at the stats…

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