Auto merge of rust-lang#15181 - lowr:patch/import-map-purge-unused, r=Veykril

Clean up `ImportMap`

There are several things in `hir_def::import_map` that are never used. This PR removes them and restructures the code. Namely:

- Removes `Query::name_only`, because it's *always* true.
  - Because of this, we never took advantage of storing items' full path. This PR removes `ImportPath` and changes `ImportInfo` to only store items' name, which should reduce the memory consumption to some extent.
- Removes `SearchMode::Contains` for `Query` because it's never used.
- Merges `Query::assoc_items_only` and `Query::exclude_import_kinds` into `Query::assoc_mode`, because the latter is never used besides filtering associated items out.

Best reviewed one commit at a time. I made sure each commit passes full test suite. I can squash the first three commits if needed.

Author: bors

crates/hir-def/src/find_path.rs

@@ -360,7 +360,7 @@ fn calculate_best_path(
                     prefer_no_std,
                 )?;
                 cov_mark::hit!(partially_imported);
-                path.push_segment(info.path.segments.last()?.clone());
+                path.push_segment(info.name.clone());
                 Some(path)
             })
         });