nightly-2024-10-10: fix: visibility for impl methods (#6261)

# Description

## Problem

Part of #4515

Apparently in #6179 I only made it work for calls like `foo::Bar::baz()`
but no checks were done for method calls (where `self` is involved).

## Summary

This PR now warns when calling private or `pub(crate)` methods, either
on structs or primitive types, when they are not accessible from the
current module.

We also now don't suggest non-accessible methods from LSP.

## Additional Context


I also removed an unused global that was introduced some time ago.

## Documentation

Check one:
- [x] No documentation needed.
- [ ] Documentation included in this PR.
- [ ] **[For Experimental Features]** Documentation to be submitted in a
separate PR.

# PR Checklist

- [x] I have tested the changes locally.
- [x] I have formatted the changes with [Prettier](https://prettier.io/)
and/or `cargo fmt` on default settings.