Rename ui_toolkit to ui and import it from the top-level

[cpsievert]

@wch not really related to this PR, but I'm not sure how/whether to fix this:

shiny run --reload examples.moduleapp.app
INFO:     Uvicorn running on http://127.0.0.1:8000 (Press CTRL+C to quit)
INFO:     Started reloader process [72330] using statreload
INFO:     Started server process [72332]
INFO:     Waiting for application startup.
INFO:     Application startup complete.
INFO:     127.0.0.1:61688 - "GET / HTTP/1.1" 200 OK
INFO:     127.0.0.1:61689 - "GET /lib/shiny-0.0.1/shiny.min.css HTTP/1.1" 304 Not Modified
INFO:     127.0.0.1:61688 - "GET /lib/jquery-3.6.0/jquery-3.6.0.min.js HTTP/1.1" 304 Not Modified
INFO:     127.0.0.1:61691 - "GET /lib/shiny-0.0.1/shiny.js HTTP/1.1" 304 Not Modified
INFO:     127.0.0.1:61697 - "GET /lib/bs3-compat-1.0/transition.js HTTP/1.1" 304 Not Modified
INFO:     127.0.0.1:61688 - "GET /lib/bs3-compat-1.0/tabs.js HTTP/1.1" 304 Not Modified
INFO:     127.0.0.1:61693 - "GET /lib/bootstrap-5.0.1/bootstrap.min.css HTTP/1.1" 304 Not Modified
INFO:     127.0.0.1:61695 - "GET /lib/bootstrap-5.0.1/bootstrap.bundle.min.js HTTP/1.1" 304 Not Modified
INFO:     127.0.0.1:61689 - "GET /lib/bs3-compat-1.0/bs3compat.js HTTP/1.1" 304 Not Modified
Exception in ASGI application
Traceback (most recent call last):
  File "/Users/cpsievert/opt/anaconda3/lib/python3.8/site-packages/uvicorn/protocols/websockets/websockets_impl.py", line 171, in run_asgi
    result = await self.app(self.scope, self.asgi_receive, self.asgi_send)
  File "/Users/cpsievert/opt/anaconda3/lib/python3.8/site-packages/uvicorn/middleware/proxy_headers.py", line 59, in __call__
    return await self.app(scope, receive, send)
  File "./shiny/app.py", line 99, in __call__
    await self.starlette_app(scope, receive, send)
  File "/Users/cpsievert/opt/anaconda3/lib/python3.8/site-packages/starlette/applications.py", line 119, in __call__
    await self.middleware_stack(scope, receive, send)
  File "/Users/cpsievert/opt/anaconda3/lib/python3.8/site-packages/starlette/middleware/errors.py", line 146, in __call__
    await self.app(scope, receive, send)
  File "/Users/cpsievert/opt/anaconda3/lib/python3.8/site-packages/starlette/exceptions.py", line 63, in __call__
    await self.app(scope, receive, send)
  File "/Users/cpsievert/opt/anaconda3/lib/python3.8/site-packages/starlette/routing.py", line 659, in __call__
    await route.handle(scope, receive, send)
  File "/Users/cpsievert/opt/anaconda3/lib/python3.8/site-packages/starlette/routing.py", line 318, in handle
    await self.app(scope, receive, send)
  File "/Users/cpsievert/opt/anaconda3/lib/python3.8/site-packages/starlette/routing.py", line 77, in app
    await func(session)
  File "./shiny/app.py", line 151, in _on_connect_cb
    session = self.create_session(conn)
  File "./shiny/app.py", line 78, in create_session
    session = Session(self, id, conn, debug=self._debug)
  File "./shiny/session.py", line 149, in __init__
    self.app.server(self.input, self.output, self)
  File "./examples/moduleapp/app.py", line 43, in server
    counter_module.server("counter1")
  File "./shiny/shinymodule.py", line 91, in server
    session_proxy = SessionProxy(ns, session)
  File "./shiny/shinymodule.py", line 71, in __init__
    self.input: InputsProxy = InputsProxy(ns, parent_session.input)
  File "./shiny/shinymodule.py", line 20, in __init__
    self._ns: str = ns
  File "./shiny/shinymodule.py", line 38, in __setattr__
    super().__setattr__(attr, value)
  File "./shiny/session.py", line 567, in __setattr__
    self.__setitem__(attr, value)
  File "./shiny/shinymodule.py", line 27, in __setitem__
    self._values[self._ns_key(key)].set(value)
  File "./shiny/shinymodule.py", line 45, in __getattr__
    return object.__getattribute__(self, attr)
AttributeError: 'InputsProxy' object has no attribute '_values'

[wch]

Thanks for the heads up about modules. I'll fix it in a separate PR.

[wch]

Looks good, but there are merge conflicts.