We might want to provide more efficient impls for check if usages
exist, limiting the search, filtering and cancellation, so let's
violate YAGNI a bit here.
Conjecture: it's impossible to use hir::Path *correctly* from an IDE.
I am not entirely sure about this, and we might need to add it back at
some point, but I have to arguments that convince me that we probably
won't:
* `hir::Path` has to know about hygiene, which an IDE can't set up
properly.
* `hir::Path` lacks identity, but you actually have to know identity
to resolve it correctly
