Method selection doesn't find enum variant constructors in other ingots

[sbillig]

Example: in some other ingot (eg core):

pub enum Option<T> {
    Some(T),
    None,
}

in a file that uses core:

use core::Option

fn f() {
    let x = Option::None // this is fine
    let y = Option::Some(10) // `Some` is not found in `Option`
}

The CandidateAssembler only collects enum variant constructors defined in the ingot of the current scope.

I'll try to fix this in the next day or two, just putting it here in case someone else runs into it.