solver3: Upgrade by source package

Implement the "APT::Get::Upgrade-By-Source-Package" option as
the classic solver does.

Here this is equally straight forward now: We enqueue optional, but
eager, clauses of the form

    foo=2 -> foo-data=2

for each sibling in the source version, assuming they are currently
installed, and the selection is not the current version.

This softly enforces upgrades of already installed siblings, but
in non-strict-pinning mode it will not affect the selection of
new packages to be installed.

A more complete solution to version selection by source package
also seems feasible, where we change the "priority" of versions
in the solver dynamically - currently they are statically evaluated.
Such that, when you select foo=2, and something installs foo-data,
foo-data would be installed in version 2 even if version 3 were its
candidate.

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