bpart: Also partition the export flag (#57405)
Whether or not a binding is exported affects the binding resolution of
any downstream modules that `using` the module that defines the binding.
As such, it needs to fully participate in the invalidation mechanism, so
that code which references bindings whose resolution may have changed
get properly invalidated.
To do this, move the `exportp` flag from Binding into a separate bitflag
set that gets or'd into the BindingPartition `->kind` field. Note that
we do not move `publicp` in the same way since it does not affect
binding resolution.
There is currently no mechanism to un-export a binding, although the
system is set up to support this in the future (and Revise may want it).
That said, at such a time, we may need to revisit the above decision on
`publicp`.
Lastly, I will note that this adds a fair number of additional
invalidations. Most of these are unnecessary, as changing an export only
affects the *downstream* users not the binding itself. I am planning to
tackle this as part of a larger future PR that avoids invalidation when
this is not required.
Fixes #57377
