4501: Querify `importable_locations_in_crate` r=jonas-schievink a=jonas-schievink
This brings the time needed to compute the `add_missing_impl_members` assist down from ~5 minutes to 20 seconds on my test workload (which is editing within an impl of a MIR [`MutVisitor`](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/mir/visit/trait.MutVisitor.html))
cc #4498
Co-authored-by: Jonas Schievink <jonasschievink@gmail.com>