discard candidates via IsInstallOk to allow override

In commit 446551c8 I changed MarkInstall to discard the candidate if the
candidate can't satisfy the dependency. This breaks interactive solvers
like aptitude which can change the candidate on-the-fly later.

In commit df77d8a5 I introduced this 'early' loop-breaking to begin with
which can't be that helpful for interactive solvers as well, but makes
perfect sense for non-interactives to stop them from exploring trees
which can't be satisfied, but it isn't perfect as ideally we would check
this before auto-installing the first dependency.

This commit therefore moves the loop into its own IsInstallOk hook so
that frontends can override this check if they want to and in exchange
removes the loop-breaking from MarkInstall itself and does it before any
dependency is installed.

Closes: 740750

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