Propagate Protected flag to single-option dependencies

If a package is protected and has a dependency satisfied only by a single
package (or conflicts with a package) this package must be part of the
solution and so we can help later actions not exploring dead ends by
propagating the protected flag to these "pseudo-protected" packages.

An (obscure) bug this can help prevent (to some extend) is shown in
test-apt-never-markauto-sections by not causing irreversible autobit
transfers.

As a sideeffect it seems also to help our crude ShowBroken to display
slightly more helpful messages involving the packages which are actually
in conflict.

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