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.

Refinement types are still able to inherit private members from
all direct parents, as was needed in pos/t2399.scala. More tests
are included for this scenario.

In short, the logic now:

 - includes direct parents of refinements,
 - otherwise, excludes privates after the first class in the
   base class sequence