Correct broken dselect logic for self-conflicting packages

Correct broken dselect logic for self-conflicting packages

The previous code seems to have had some confusion over the precedence
of '&&' vs ':'. The new rule is that a Conflicts is violated if the
target package is installed or to be error-preserved ie
would_like_to_install() != 0 - except that we disregard packages which
directly Conflict with themselves.