Fix cross-ingot inherent method resolution

[sbillig]

This wasn't working before:

use core::Option // where `core` is a different ingot from this file

fn f() {
  let x = Option::Some(10) // failed to resolve `Option::Some` variant constructor
  x.unwrap() // failed to resolve `unwrap` method on `Option` type
  ...
}

• When a function call expression "callee" path resolves to a tuple enum variant, we now immediately resolve to that variant constructor fn, instead of using select_method_candidate_for_path to find the enum constructor (which was searching the current ingot only and thus failing to resolve).
• When resolving a method call, we now search for impl blocks in the ingot in which the receiver type is defined instead of in the current ingot.

I incidentally added some stuff to library/core to test this with realistic code.