External interface change: PP: Full <> and "" semantics for the Includer.

External interface change: PP: Full <> and "" semantics for the Includer.

Any previous use would only be for "", which would probably mean changing

    include(...)  -> includeLocal(...)

See comments about includeLocal() being an additional search over
includeSystem(), not a superset search.

This also removed ForbidIncluder, as
 - the message in ForbidIncluder was redundant: error results were
   already returned to the caller, which then gives the error it
   wants to
 - there is a trivial default implementation that a subclass can
   override any subset of (I still like abstract base classes though)
 - trying to get less implementation out of the interface file anyway