7295: Share import_assets and related entities r=matklad a=SomeoneToIgnore
Part of https://github.com/rust-analyzer/rust-analyzer/pull/7293
Addresses https://github.com/rust-analyzer/rust-analyzer/pull/7293#issuecomment-
761569558
Prepares `import_assets` and related to be used later for the trait fuzzy importing.
Also moves fuzzy imports into a separate completion module and renames them, as suggested in https://github.com/rust-analyzer/rust-analyzer/pull/7293#discussion_r558896685
Co-authored-by: Kirill Bulatov <mail4score@gmail.com>