Only show completions for prelude items when not in a module qualifier

[GearsDatapacks]

I noticed that when the LSP provides autocomplete for types and values after a module select, e.g.

pub fn main() {
  let x = option.|
}

It was including completions for prelude values, like Ok, which would not be valid after option.
This PR fixes that behaviour

[lpil]

Oh much better! Thank you!