check for conflicting selected packages before selecting a package

check for conflicting selected packages before selecting a package
instead of after selecting it, to avoid having to unselect package
in case of conflict (which incorrectly unselected packages with
unsatisfied dependencies as well, making the following backtrack_selected
call behave wrongly as it simply noticed that the package is no longer
required)

Here's the second patch again, now with the function comment updated
and added _remove_all_rejected_from() call so that prerejections are
still cleared (this was previously called from the
removed disable_selected() call). Testsuites pass.

Patch by Anssi Hannula, fixes #57224

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