Auto merge of rust-lang#18050 - rust-lang:davidbarsky/push-uyvtlsvoqrxw, r=Veykril

assist: ensure `replace_qualified_name_with_use` applies to the first path segment

This change helps a bit with the discoverability of `replace_qualified_name_with_use`. Specifically, it ensures that a cursor on the first path segment (e.g., `$0std::fmt::Debug`, where `$0` is the cursor) would result in an import along the lines of `use std::fmt;` and `fmt::Debug;` at the usage sites.

Author: bors