solver3: Only enqueue shared dependencies at the package level

Dependencies shared by all versions are enqueued at the package
level, so avoid enqueuing duplicates at the version level. This
presumably has no meaningful impact on performance, potentially
a negative performance impact on some workloads as we now need
to find the duplicates again; it can become useful when there
is a lot of backtracking.

More importantly though this improves error messages, because
now we can say that "all versions of foo depend on X", rather
than saying "foo=1 depends on X" and you are left wondering
why we did not select "foo=2".

In this commit though, improved error messages are not implemented,
they depend on redesigning the reason tracking to use clauses.

Also the rationale tracking includes a lot more dependencies of
the form "pkg:arch=version -> pkg:arch" which are annoying. Improved
error messages should fold them into one node.

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