Move completions to DeclId

[sophiajt]

Description

This will bind the name after the @ for custom completions to a universal DeclId that can later be used.

This should allow you to export externs and use completions that are inside the module.

Fixes #4785

Tests

Make sure you've run and fixed any issues with these commands:

• cargo fmt --all -- --check to check standard code formatting (cargo fmt --all applies these changes)
• cargo clippy --all --all-features -- -D warnings -D clippy::unwrap_used -A clippy::needless_collect to check that you're using the standard code style
• cargo build; cargo test --all --all-features to check that all the tests pass

[kubouch]

This fixes it, thanks!