Merge #6351

Merge #6351

6351: Organized completions r=popzxc a=popzxc

This PR continues the work on refactoring of the `completions` crate.

In this episode:

- Actual completions methods are encapsulated into `completions` module, so they aren't mixed with the rest of the code.
- Name duplication was removed (`complete_attribute` => `completions::attribute`, `completion_context` => `context`).
- `Completions` structure was moved from `item` module to the `completions`.
- `presentation` module was removed, as it was basically a module with `impl` for `Completions`.
- Code approaches were a bit unified here and there.

Co-authored-by: Igor Aleksanov <popzxc@yandex.ru>