SI-7475 Private members are not inheritable

Exclude them from superclasses in `findMember` and in
`OverridingPairs`.

The odd logic in `findMember` that considered whether the
selector class was owned by the owner of the candidate private
symbol dates back to 2007 (bff42682b), but does not appear to
have any relationship to the spec.

One existing test was modified to reflect the stricter
rules: a private member is not contributed to a refinement
type, even when selecting from it within the class that defines
that private member.