do IsInstallOk call in MarkInstall unconditionally

Hooked checks could be influenced by AutoInst as a lot can happen
between a call without and one with this bit set. The real cache-hit
check is above this call already. Individual hooked checks can then
inspect the state if they want to cache. Calling them multiple times
shouldn't be a problem either way.
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