become an externally usable package but still remains within the main sqlalchemy parent package.
in this system, we use kind of an ugly hack to get the noseplugin imported outside of the
"sqlalchemy" package, while still making it available within sqlalchemy for usage by
third party libraries.

that change object state as these aren't intended for public
use.

and the entire related system of alternate
class implementation is now moved out
to sqlalchemy.ext.instrumentation.   This is
a seldom used system that adds significant
complexity and overhead to the mechanics of
class instrumentation.  The new architecture
allows it to remain unused until
InstrumentationManager is actually imported,
at which point it is bootstrapped into
the core.