Better handling of completions that shadow in scope entities

So to clarify, I would see the following possible action items:

• detect when a trait autoimport would shadow an inherent method and downrank those completions

• detect when a trait autoimport would shadow an inherent method and instead complete a fully qualified method call (e.g. foo.borrow_<|> -> std::borrow::BorrowMut::borrow_mut(foo))

• detect when there's an inherent method currently shadowed by a trait method, and complete the fully qualified method call instead (e.g. when there's use std::borrow::BorrowMut; already, provide RefCell::borrow_mut(foo) as a completion)

• when using the UFCS -> method call assist, detect if the method call would be shadowed and remove e.g. the BorrowMut import automatically (maybe only if it's unused)

Originally posted by flodiebold in #13786

Begin with the four proposed action items and the linked discussion in rust-analyzer issue #13786. Investigate the completion and UFCS method-call assist behavior around trait autoimports, inherent methods, shadowing, and fully qualified calls. Done should mean the agreed shadowing cases are handled consistently, with coverage for the examples described here.