This isn't really needed currently (we'd know, the symptom is a
RecursionError at import time), but it's a genuine bug fix for a
problem I ran into while implementing python-triogh-316. And since that PR seems
to be stalled at the moment, I want to get this in so it doesn't get
forgotten.

In particular, this should make it so PyCharm's completion starts
working.

Fixes python-triogh-314.

Apparently PyCharm gives up on parsing __all__ entirely if it sees
__all__ += foo.__all__, so move the static trio.__all__ entries that
we want to be visible to static analyzers into their own dedicated
submodule.

See: python-trio#314 (comment)