Throw an error with passed slots that are not top-level

[ElianHugh]

Passing non-top level slots to components doesn't really make sense, and should probably through an error.

E.g.

x <- component(
    template = function(ns) {
        shiny::tagList(
            shiny::tags$slot(),
            shiny::tags$slot(name = "a")
        )
    }
)

# the following would resolve to both slot 1 and slot 2
x(
    shiny::div(
        shiny::p(slot = "a")
   )
)