For Express apps, confusing error if `input.x` is accessed without importing `input`

[wch]

If the app tries to access input without importing it from shiny.express, it prints the following confusing error message:

RuntimeError: 'builtin_function_or_method' object has no attribute 'x'

Example app:

from shiny import reactive
from shiny.express import ui
# from shiny.express import input

ui.input_action_button("x", "Click me")

@reactive.Effect()
@reactive.event(input.x)
def foo():
    print("Clicked button")

This is because input is a builtin function in Python, but it doesn't have an attribute named x. So although the error is correct, it is a bit confusing.

I think that while evaluating the code, we could inject a different object named input that will result in a better error message.